What goes wrong
Keep each size in its own shelf slot.
Packaging Suppliers lists Mailing bags for this line. The mailroom runs out of one size first. That size is the reorder, not a mixed assortment.
The check before you reorder
Choose the mailer that fits the goods. A much larger bag folds and looks like a different pack.
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.