When I first started selling on eBay I had this argument with my accountant.
I am selling postage for the cost of the stamp so I shouldn't be charging VAT on it was my argument however my accountant disagreed.
IF they are using a VAT exempt postage option (not all RM services are) they will still have to charge you VAT as they are not selling you a stamp, they are selling you a service that includes the use of that stamp. If they sold a book a stamps to you then they wouldn't need to charge VAT on the book of stamps but they would on posting the stamps to you as it is a service.