Tamper-Evident Postal Packaging Our M4 cardboard envelope mailer is one of the larger sizes in the Lil "M-Spec" range, purpose built for A3 documents, larger prints, and multi item orders that need more protection than a poly mailer but don't need a full box. Made from strong corrugated kraft cardboard, the M4 offers box form rigidity in a slim, letterbox friendly format. Pre folded flaps and ghost creases mean fast, consistent packing every time, and the peel and seal tear strip closure means you can seal orders without tape. At 322 x 384mm flat packed, the M4's dimensions already exceed Royal Mail's large letter size limit, so it ships under the small parcel tariff whether sent flat or expanded (300 x 252 x 70mm), both within the small parcel maximum of 450 x 350 x 160mm. (See our guide to Royal Mail postage tariffs for the full breakdown.) Our peel and seal closure uses a high tack adhesive that bonds permanently on contact, unlike lower security seals that can be carefully peeled back and reapplied without obvious signs of tampering. Once sealed, our mailers are tamper evident: any attempt to open the parcel in transit leaves a clear, visible mark, giving your customers confidence that what arrives is exactly what was sent. Ideal for: A3 documents, larger prints and artwork, multi item orders, lightweight clothing, and other larger flat or lightly bulky items. Lil Packaging is Europe's largest independent producer of ecommerce postal packaging. See more of our M-Range Cardboard Mailers. Product Specification: Size: 322 x 384mm (flat pack) / 300 x 252 x 70mm (expanded) Material: Corrugated kraft cardboard Closure: Peel & seal, tamper evident Postal fit: Royal Mail Small Parcel (flat and expanded, exceeds large letter limit) Certifications: FSC®, Plastic Free, Zero Carbon Why choose the M4: 100% recyclable, FSC® certified, plastic free Zero Carbon manufacturing (UK made) Peel and seal closure, no tape needed Reinforced corners reduce in transit damage
