What goes wrong
Reorder the size from the six, not a made-up dimension.
Packaging Boxes lists Crash lock boxes for this line. The category describes the board as 150K/T, recyclable and biodegradable, and sized towards Royal Mail small parcel guidelines. It lists six sizes.
The check before you reorder
Pick from the six sizes. If none is close, use single wall rather than a crash lock that is mostly air.
Store the six sizes as six locations, not one crash-lock pile.
Dispatch on that order
At Packaging Boxes, Crash lock boxes does not change the cutoff. Orders by 3pm leave the same day. Under £50 ex VAT the delivery is still available. It is just not the free threshold.