a fee to pay from Royal Mail for £3.50 is either for insufficient postage on a small parcel and or they have used non barcoded stamps. If you want to post a picture of the package front (removing your personal details) we will be able to tell you why you got that fee from RM. 

However aside ebay wont step in, this is between you and the seller. What you should have done is not pay the fee, and then opened an INR case and you would have had a full refund.