Stocked cartons
Stocked boxes can be returned within 14 days of purchase for a refund, credit or exchange. Call +44 (0)208 386 6960 and arrange it. A bundle of cartons sent back without a call is not the process on the returns page.
A size printed or made for the order is a different case. Read that page before the goods leave you.
Damage found later
A defect that was not obvious on delivery needs notice within 14 days of finding it, and no later than 30 days from delivery.
The next box order is separate. It still has to be in by 3pm.