The order in writing
The purchase order and the pro forma invoice, with the size grid, the sealed sample reference and the pack specification attached.
A purchase orderA pro formaAttachments
Everything agreed so far now goes into two documents. If it is not in them, it is not agreed, whatever was said on a call.
Your purchase order carries the piece by size by colour table, the price per piece, the currency, the delivery term, the ship date, the sealed sample reference, the size grid with its tolerances, the pack specification, the tests and their required grades, the inspection standard and the acceptable quality level, and what happens if the goods fail. Attach the size grid and the pack specification as files rather than describing them.
The mill's pro forma invoice comes back with the same figures, the bank details in the mill's own company name, and the payment schedule. Read the two against each other line by line. A pro forma that differs from your order in any figure is a renegotiation, not a formality.
Three clauses worth insisting on in home textiles specifically. A shade band, so that acceptable variation is defined rather than argued. A shortage and overage tolerance, because cloth is cut and there is always a small difference. And a clause naming who owns the sealed sample and where it is kept, because in a dispute the party holding it is the party who wins.