Amazon cartons follow a published maximum. A general cube does not become one by being about the right size. This page stays with crash lock boxes, next to white postal boxes.
Packaging Boxes keeps Crash lock boxes apart from a tape-closed cube. The category describes the board as 150K/T, recyclable and biodegradable, and sized towards Royal Mail small parcel guidelines. It lists six sizes.
What this collects
A crash lock box opens into shape and has a peel-and-seal strip.
white postal boxes is the neighbour on this list. White postal boxes are the white finish for a parcel that is meant to look like post, not like a warehouse carton. It is still a different range from crash lock boxes.
Before the order goes in
Pick from the six sizes. If none is close, use single wall rather than a crash lock that is mostly air.
TradePro at Packaging Boxes, for trade businesses, starts at 5% off and goes up to 10% when you spend over £1,000 a month, and it adds BACS. It does not change which Crash lock boxes you needed.