Measure before you open the range
Pick the envelope from the document size, not from the nearest mailer.
A board-back envelope keeps a document flat. The page for Board-back envelopes is where Packaging Suppliers shows that line.
The published limit, if there is one
Board-back envelopes are a separate list from plain envelopes and from mailing bags.
A mailer folds. That is acceptable for a tee shirt and wrong for a certificate or a photo that must not crease.
Write the size down
Reorder the board-back size, not “envelopes”.
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 Board-back envelopes you needed.
Open Board-back envelopes when you already know this line. The pack count is on the product. Packaging Suppliers will still dispatch an order placed by 3pm on the same day.