You should use the postage supplied via ebay to keep your seller protection. If you use a different service, outside of ebay, you must generate the original label and change the tracking number to mark the item as dispatched.
If ebay supplied an Evri label but the buyer wants RM the best thing to do (especially if item is not cheap) is to cancel/refund the transaction with the buyer's agreement (via the dropdown actions menu arrow for the item on your orders list) and relist with correct postage for the buyer to buy and pay again.
