By the Samz Packaging team, Bradford, UK · Updated November 2026

Mailing bags vs boxes: the main differences

Both do the same job, getting an order to the customer in one piece, but they suit different products. The right choice protects the item, keeps postage down and gives a good first impression.

Royal Mail size limits you need to know

For UK sellers, the size of the packed parcel often decides the price. Royal Mail’s published limits for two common formats are:

When mailing bags are the right choice

• Clothing, knitwear, scarves and accessories
• Bedding, towels and other soft textiles
• Flat, lightweight, non-fragile items
• Orders where keeping postage costs low matters most

Pick a size that holds the item without strain, with a little room to fold the flap and seal it cleanly. A bag that is stretched tight is more likely to burst.

When you should use a box instead

Choose a box for glass, ceramics, electronics, candles, gift sets and anything that would be damaged by being squashed. If the item has a hard edge or a corner that could push through a bag, a box is the safer choice. A box also stacks neatly in a courier van, which cuts the chance of crush damage.

Seal cartons properly with good packing tape. Our guide to which packaging tape to use explains the H-seal method that keeps boxes closed in transit.

Choosing a grey mailing bag size

Our grey mailing bags are lightweight and discreet, and come in a wide range of sizes from small bags for accessories up to large bags for bulky soft goods. The size of each bag is shown in its product name, so you can match it to your product:

• Small accessories and jewellery: see the 4.5 x 7 and 6.5 x 9 sizes.
• T-shirts, tops and folded clothing: see the 12 x 16 and 14 x 21 sizes.
• Coats, bedding and bulky items: see the 28 x 34 size and above.

Browse the full mailing bags range to compare every size.

How to pack a mailing bag

1. Fold or roll the item so it lies flat and does not bulge.
2. Add a layer of tissue or an inner bag if you want to keep the item clean or protect a delicate fabric.
3. Do not overfill the bag. Leave room to fold the flap and seal it.
4. Press the seal firmly along its whole length and check it before posting.
5. Attach the label flat so it can be scanned easily.

Mailing bags vs boxes FAQs

Are mailing bags cheaper to post than boxes?

Often, yes. A bag is lighter and flatter than a box, so a parcel in a bag is more likely to fit a cheaper Royal Mail size band. The saving depends on the final size and weight of the parcel.

What can I send in a mailing bag?

Soft, non-fragile items such as clothing, scarves, towels and bedding are ideal. Avoid sending rigid, fragile or sharp items in a bag, because it offers little crush protection.

What are the Royal Mail Large Letter and Small Parcel limits?

Royal Mail’s published limits are 353 x 250 x 25 mm and 750 g for a Large Letter, and 450 x 350 x 160 mm and 2 kg for a Small Parcel. Check royalmail.com for the latest limits.

Which size mailing bag should I buy?

Choose a bag slightly larger than the folded item so you can seal the flap without straining the bag. The size of each of our grey mailing bags is shown in the product name.

Should I use a box or a bag for fragile items?

Use a box. A bag does not protect against crushing or impact, so fragile items, glass and electronics should always go in a sturdy box with suitable padding.

More UK packaging guides