Postal Boxes

How Royal Mail Pricing in Proportion Works

Royal Mail’s Pricing in Proportion (PiP) system charges postage based on the size and weight of an item rather than weight alone. Items fall into tiers (Letter, Large Letter, Small Parcel, Medium Parcel) based on their physical dimensions. A box that fits within the Large Letter dimensions, for example, posts at the Large Letter rate even if a smaller item inside leaves room to spare.

This matters commercially because a box that is even slightly over the Large Letter dimensions falls into Small Parcel pricing, which costs noticeably more. Buying boxes that are engineered to the exact PiP keeps your postage costs predictable. Our postal boxes are made to sit just within each PiP tier, with the depth and footprint calibrated against the Royal Mail dimension limits.

Large Letter Postal Boxes

Large Letter postal boxes are 20mm deep, sitting just within the Royal Mail Large Letter depth limit. They are suited to flat, lightweight items such as paperbacks, photo prints, documents, phone cases, jewellery in flat presentation boxes, small fashion accessories and cosmetic samples. All five sizes are packed in 50s and supplied flat-packed.

Postal Boxes Small (Large Letter PiP) is the smallest in the range, suited to small flat items such as jewellery, phone cases and small fashion accessories.

Postal Boxes C6 (Large Letter PiP) sized to fit A6 paper, postcards and small documents. Suited to photo prints, greetings cards and small product packaging.

Postal Boxes DL (Large Letter PiP) sized to fit a DL envelope (A4 folded twice). Useful for compliment slips, vouchers and slim product cards.

Postal Boxes C5 (Large Letter PiP) sized to fit A5 paper. A common choice for small books, magazines, A5 documents and similar flat items.

Postal Boxes C4 (Large Letter PiP) sized to fit A4 paper unfolded. The largest Large Letter size in the range, suited to A4 documents, brochures, slim books and larger photo prints.

Small Parcel Postal Boxes

Small Parcel postal boxes are deeper than the Large Letter range and suit items that need more vertical space inside the box. They sit within Royal Mail’s Small Parcel PiP tier so postage stays predictable, and they are useful where a Large Letter box would not fit the item even though the footprint is in the right range. All three sizes are packed in 25s and supplied flat-packed.

Postal Boxes Mini (Small Parcel PiP) is the smallest Small Parcel size. Suited to small product boxes that are too deep for Large Letter, including chunky jewellery boxes, small candles, soap bars and short cosmetics.

Postal Boxes Midi (Small Parcel PiP) steps up the footprint and depth. Suited to small cosmetics sets, hardback books, branded retail boxes for bottled or jarred products, and gift sets.

Postal Boxes Maxi (Small Parcel PiP) is the largest in the range at 420 x 340 x 73mm. Suited to A4 format hardbacks, photo books, larger cosmetics sets and packaged homeware items that need both width and depth.

Why Use Cardboard Postal Boxes for Royal Mail and Couriers?

A purpose-built cardboard postal box gives flat items a rigid protective shell that a padded envelope or simple wrap cannot match. Documents, prints, books and similar items arrive without bent corners or creases.

Cardboard postal boxes also signal professionalism on the doorstep. A branded or plain cardboard box with a clean fold reads as a considered, deliberate package, where a padded mailer can feel more incidental. For e-commerce, subscription and gifting brands shipping flat or low-depth products, the Large Letter PiP range is often the most cost-effective option in the market, particularly compared to upgrading to Small Parcel postage to use a deeper non-PiP box.

All of our postal boxes are made from FSC or PEFC approved sustainable cardboard, are recyclable through standard household recycling, and are supplied flat-packed to reduce storage space and the carbon footprint of delivery to your premises.

Postal Box FAQs

Q: What is the maximum size for Royal Mail Large Letter postage?

Royal Mail Large Letter items must fit within 353mm x 250mm x 25mm and weigh no more than 750g. Our Large Letter postal boxes are 20mm deep, sitting just within the 25mm Large Letter depth limit so a correctly packed item posts at the Large Letter rate.

Q: What is the difference between Large Letter and Small Parcel postal boxes?

The two tiers differ in depth and Royal Mail postage rate. Large Letter postal boxes are 20mm deep, suited to flat items like documents, books and prints. Small Parcel postal boxes are deeper (the Maxi is 73mm), suited to bulkier items. Large Letter postage is cheaper than Small Parcel, so use the Large Letter box whenever the item fits.

Q: Do the boxes need tape to assemble?

The boxes are pre-cut and scored for hand assembly. They fold into shape and tuck closed without requiring tape or glue for the structure itself, although a strip of tape can be used to secure the closure for transit on heavier or higher-value items.

Q: Are the postal boxes recyclable?

Yes. All Forms Plus postal boxes are made from FSC or PEFC approved sustainable cardboard and can be recycled through standard household cardboard recycling after use.

Why Buy Postal Boxes from Forms Plus?

Forms Plus have supported businesses with their printing and packaging requirements since 1981. We work hard to make sure our standard packaging lines perform to the same level as the brand-name equivalents. We stock the full eight-size postal box range with competitive pricing and in stock, for same day despatch.

Free samples are available on selected products so you can test the fit, fold and finish before committing to a bulk order. If you are not sure which size suits your products, our team can advise.

We also supply a full range of complementary packaging products, including Crash Lock Boxes, Book Wraps, Padded Envelopes, Mailing Bags and Labels, so you can keep all of your packaging supplies in one order.

Request a Free Sample or Contact Our Team if you would like help choosing the right postal box for your needs.