What goes wrong
Packs are the unit. Read how many bags are in the pack.
Packaging Suppliers lists Mailing bags for this line. Count the soft dispatches of that size.
The check before you reorder
They are also called shoe bags. The category says they are European made from recycled film and 100% recyclable, and opaque so the contents are not visible.
Keep each size in its own shelf slot.
Dispatch on that order
TradePro at Packaging Suppliers, 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 Mailing bags you needed.