What goes wrong
Grey mailers sit with kraft tape or a polythene-suitable seal, not with void fill.
Packaging Suppliers lists Mailing bags for this line. Order grey mailers for the unbranded outer. PayPal branded bags are a separate range, in 250 x 350 mm, 305 x 405 mm and 425 x 600 mm.
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.