RMA (return merchandise authorisation) is the approval and reference number a seller gives a customer before they send an item back, so the return can be tracked and matched to the order.
A typical RMA process is: the customer asks to return, you check the order and reason, you issue a reference and return instructions, the parcel arrives, you inspect it, and you refund or replace.
In the UK, the Consumer Contracts Regulations 2013 let most online buyers cancel within 14 days of receiving goods, with no reason needed. They then have 14 days to send the goods back, and you must refund within 14 days of getting them back or receiving proof of return, including standard outbound delivery. The customer pays return postage only if you told them before purchase. Faulty goods are covered separately by the Consumer Rights Act 2015, including a 30-day right to reject for a full refund. This is a general guide, not legal advice.
For wording, see the return instructions template.