Gusset Envelope Guide: Capacity Mailers for Books and Thicker Items

A gusset envelope has extra material folded into its sides that opens out to give it depth, so it can carry items that are too thick for a flat envelope. Capacity mailers are the rigid board version. Pack Solution stocks them in four sizes, M1 to M4, each with a 70mm expanding gusset and packed 50 to a box.

They suit paperbacks, hardbacks, catalogues, brochure bundles and small boxed products. Before you order, it helps to know which size takes A4, how a mailer compares with a padded envelope or a box, and what the extra depth does to your postage.

Quick Picks: Gusset Envelope and Alternatives

These three cover most thick items you are likely to post. Start with the capacity mailer and keep the other two for the items that suit them better.

Best for books and boxed items up to 70mm thick

Capacity Mailer Envelopes

Rigid board envelopes with a 70mm expanding gusset that still look flat and neat when they arrive.

• Four sizes, M1 (219 x 155 x 70mm) to M4 (312 x 250 x 70mm)
• M4 takes A4 items
• Boxes of 50

From £30.00 ex VAT (£36.00 inc VAT)

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Best for small, light items that need cushioning

White Bubble Padded Envelopes

White self-seal envelopes with a bubble lining that absorbs knocks in transit.

• 10 sizes, EP1 (100 x 165mm) to EP10 (350 x 470mm)
• Self-seal closure
• Boxes of 100

From £8.00 ex VAT (£9.60 inc VAT)

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Best for items deeper than a capacity mailer

Crash Lock Boxes

Cardboard boxes with a base that locks as you open them, so only the top needs tape.

• CL-A (100 x 100 x 70mm) to CL-K (426 x 300 x 300mm)
• Supplied flat
• Minimum 5 per size

From £0.59 ex VAT (£0.71 inc VAT)

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The capacity mailer envelopes are the main pick for books and boxed goods. White bubble padded envelopes suit small, light items that need some cushioning. Crash lock boxes take over when an item is deeper than 70mm or needs a box around it.

Key Facts

• Capacity Mailer Envelopes come in M1 (219 x 155 x 70mm), M2 (250 x 165 x 70mm), M3 (285 x 205 x 70mm) and M4 (312 x 250 x 70mm).
• Every capacity mailer size has a 70mm expanding gusset and comes in a box of 50.
• The M4 takes an A4 item (210 x 297mm), and the M3 is too short for A4.
• Board Back Envelopes (Do Not Bend) come in C4, C5 and C6 for flat documents and prints.
• White Bubble Padded Envelopes run from EP1 (100 x 165mm) to EP10 (350 x 470mm) in boxes of 100.
• Crash Lock Boxes run from CL-A (100 x 100 x 70mm) to CL-K (426 x 300 x 300mm), with a minimum of 5 per size.

What Is a Gusset Envelope?

A gusset envelope is an envelope with folded sides or a folded base that open out to give it depth. A flat envelope holds paper, while a gusseted one can hold an object.

Gussets come in two broad forms. Paper and manilla gusset envelopes have pleated sides and turn up in offices for thick files and reports. Board capacity mailers use rigid board with a gusset that expands to a set depth, in this case 70mm. The board keeps its shape, so a book or boxed item is held firmly, where a soft mailer would give way at the corners.

The finished mailer still looks like an envelope. It is sent out flat, folds open around the item and arrives looking clean, which is why booksellers, retailers and online shops use them for orders where first impressions count.

Where a Gusset Envelope Works Well

• Paperbacks and hardbacks, one at a time or several together.
• Catalogues, brochure packs and printed bundles too thick for a flat envelope.
• Small boxed products such as cosmetics, accessories and gift sets.
• Folded clothing and accessories that need a firmer shape than a bag gives.

For thin paperwork, a gusset is more envelope than you need. Our guide to A4 and C4 envelopes for posting documents covers flat paperwork and the sizes that fit it, and the postal envelopes guide compares the wider range.

Capacity Mailer Sizes: M1 to M4

Choose a capacity mailer by the length and width of your item, then check its thickness against the 70mm gusset. All four sizes share the same depth, so only the footprint changes.

Measure the item at its widest points, including any retail box or wrap, and leave a little room on the length and width so the flap closes without strain. A tight fit puts stress on the corners and the closure.

If an item sits between two sizes, go up. You can fill a small gap with a sheet of paper, but an envelope that is too small will not close. Sellers with mixed stock often keep two sizes on the bench: an M1 or M2 for single paperbacks and an M4 for A4 items and multi-book orders.

A4 Gusset Envelopes and C4 Sizes

For a4 gusset envelopes, choose the M4 capacity mailer. At 312 x 250 x 70mm it is larger than an A4 sheet (210 x 297mm) and has the depth for folders and books, while the M3 at 285mm long is too short for A4.

C4 is an envelope size, and a C4 envelope measures 324 x 229mm, which takes an A4 sheet flat. In office stationery, c4 gusset envelopes are usually paper envelopes of that footprint with pleated sides, made for thick reports. Pack Solution does not list a paper C4 gusset envelope. For the same jobs we stock the M4 capacity mailer for thick items and C4 board backed envelopes for flat A4 documents.

Best for flat A4 documents and prints

Board Back Envelopes (Do Not Bend)

A stiff board back and a printed Do Not Bend warning keep flat contents from creasing.

• C4 (324 x 229mm), C5 and C6 sizes
• C4 and C5 in boxes of 125
• C6 in boxes of 250

From £12.00 ex VAT (£14.40 inc VAT)

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The Board Back Envelopes (Do Not Bend) are the right choice when the contents are flat and must not crease. They have a stiff board back and a printed Do Not Bend warning, in C4 (324 x 229mm), C5 (229 x 162mm) and C6 (162 x 114mm). C4 and C5 come 125 to a box and C6 comes 250 to a box. They have no gusset, so they suit certificates, prints and photos rather than books.

Which One for Thick A4 Paperwork?

A ring binder, a bound report or a bundle of brochures falls between the two. If it is flat but thick, the M4 capacity mailer holds it without bending. If it is a few sheets that must stay pristine, the C4 board back envelope does the job with less bulk. Paperwork that travels on the outside of a parcel, such as invoices and delivery notes, belongs in a wallet, and our documents enclosed wallets guide explains the sizes.

Gusset Envelope vs Padded Envelope vs Box

Use a gusset envelope for firm items up to 70mm thick that need a neat finish, a padded envelope for small items that need cushioning, and a box for anything deeper or fragile.

A padded envelope is lighter and softer. Its bubble lining absorbs knocks, and the self-seal flap closes with no tape. The weak point is shape. A soft envelope wraps itself around the item, so the corners of a book can get bent or poke through on a long journey. For a paperback that has to arrive in sellable condition, the rigid board of a capacity mailer holds its corners better.

A box gives the most depth and room for void fill. A crash lock box has a base that locks into place as you open it, so only the top needs tape. Use one when an item is deeper than 70mm, when it is fragile and needs wrapping, or when several items have to travel together without shifting. For a wider look at soft and rigid packaging, read padded envelopes vs postal boxes.

Large Letter or Parcel? Checking the Thickness

A filled capacity mailer that is more than 25mm thick will not go as a Royal Mail Large Letter, so most gusseted items go at parcel rates. Check the current size and weight limits with your carrier before you set your postage charges.

The Large Letter format has a maximum thickness of 25mm, along with length, width and weight limits. A gusset envelope exists to carry items thicker than that, so plan on small parcel postage for most orders. A slim paperback in an M1 might stay under the limit, but measure the sealed envelope, since the board itself adds a little thickness.

Weight matters too. A rigid board mailer weighs more than a padded envelope, and that can tip a heavy book into the next band. Weigh a packed sample of each product you sell and note the figure, so you set shipping once and do not have to guess at the counter.

1. Pack one sample of the item in the mailer you plan to use.
2. Close it and measure the thickest point with a ruler.
3. Weigh the sealed envelope.
4. Compare both figures with your carrier's current limits.
5. Write down the service and mailer size for that product so every order goes out the same way.

How to Pack a Gusset Envelope

Open the gusset fully, lay the item flat in the centre, fold the sides in around it and close the flap. Once you have done a few, it takes seconds.

1. Pick the size by the item's length and width, with a little room to spare.
2. Open out the mailer and push the gusset to its full depth.
3. Slide the item in flat, with the spine or heaviest edge towards the base.
4. If there is a gap, add a folded sheet of paper so the item cannot slide.
5. Close the flap, and run a strip of tape along it if the item is heavy.
6. Put the address label on the flat front, clear of the folds.

Books with a glossy cover can scuff against board, so wrap them in a sheet of paper or a clear bag first. For boxed products, check the retail box is clean before it goes in. A customer often judges the product by how it looks the moment the mailer opens.

Stack several books with the largest at the bottom, so the pile sits level and the gusset closes evenly. If the stack is thicker than 70mm, split it across two mailers or move it into a box.

Buying Capacity Mailers in Bulk

Capacity mailers are sold by the box of 50, and the price per box falls as you order more boxes. Buy the size you use most in quantity and keep a single box of the others.

Prices on the site are shown ex VAT, with VAT added at checkout. Orders placed by 2pm Monday to Friday are dispatched the same working day, and Royal Mail Tracked 48 delivery is free on every order with no minimum. Unused items can go back within 30 days of delivery, and the refund policy has the details.

To work out how many boxes you need, count last month's orders that would have gone in each size and allow for growth. The mailers arrive flat, so a few weeks of stock fits on one shelf.

Gusset Envelope Checklist

• Measure the item's length, width and thickness before choosing a size.
• Use the M4 for anything A4 and the M1 for A5 items.
• Keep the packed thickness within the 70mm gusset.
• Wrap glossy covers so they do not scuff.
• Fill small gaps so the item cannot slide.
• Measure and weigh a sealed sample to pick the right postage.
• Move fragile or deeper items into a box with void fill.
• Use a board back envelope for flat documents that must not crease.

Choosing Your Gusset Envelope

For most sellers, the M4 covers A4 items and multi-book orders and the M1 or M2 handles single paperbacks. Add a box of padded envelopes for small, light items and a stock of crash lock boxes for anything deeper than 70mm. Every size mentioned here is in the envelopes range and the boxes collection.

Frequently Asked Questions

What is a gusset envelope used for?

A gusset envelope carries items too thick for a flat envelope, such as books, catalogues, brochure bundles and small boxed products. The gusset folds out to give the envelope depth. Capacity Mailer Envelopes are rigid board gusset envelopes that open to 70mm deep, so they keep a firm shape around the item and look neat on arrival.

Which gusset envelope fits A4?

The M4 capacity mailer, at 312 x 250 x 70mm, is larger than an A4 sheet (210 x 297mm), so it takes A4 books, folders and reports. The M3 is 285mm long, which is too short for A4. Check the thickness too, because the gusset opens to a maximum of 70mm.

Do you sell c4 gusset envelopes?

No. C4 gusset envelopes are usually paper envelopes with pleated sides, and Pack Solution does not list them. For thick A4 items the M4 capacity mailer does the same job with a rigid board body. For flat A4 documents, the C4 board back envelope (324 x 229mm) takes an A4 sheet without folding.

How deep do capacity mailers expand?

All four sizes, M1 to M4, have a gusset that expands to 70mm deep. The depth is the same across the range, so you choose the size by the length and width of your item. Each size is sold in a box of 50 envelopes, supplied flat.

Can I send a gusset envelope as a Large Letter?

Usually not once it is filled. Royal Mail's Large Letter format has a maximum thickness of 25mm, and a capacity mailer is built for items thicker than that. A thin item might stay within the limit, so measure and weigh the sealed envelope and check the current limits with your carrier.

Should I use a gusset envelope or a padded envelope?

Use a gusset envelope for firm items like books and boxed products that need to keep their shape. Use a padded envelope for small, light items that need cushioning. White Bubble Padded Envelopes come in ten sizes from EP1 to EP10, while capacity mailers come in four sizes with a 70mm gusset.

Shop for This Job

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Related Guides

• A4 Envelopes for Posting Documents Flat: C4 Sizes and Board Backs
• Documents Enclosed Wallets: Sizes, Plain or Printed and How to Use Them
• Postal Envelopes Guide Board Back Capacity
• Padded Envelopes vs Postal Boxes for Small Items

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