Request for extra postage from royal mail

[joyvalcarcel]

My buyer paid £2.94 for a post office 48 tracked service. The parcel is light and thin and packed in a plastic envelope. It has not been delivered as the post office is now requesting a further £3.50 to deliver it which is quite reduculous. Trying to contact customer service eBay but no luck and no idea what to do. Payment is not processed either as parcel not delivered 

[john1297576]

£3.50 is the amount charged by Royal Mail for underpaid postage on a Small Parcel.

You just say light and thin, but it is the overall measurements that decide whether it is Letter, Large Letter or parcel. If over 25cm x 35.3cm it is a parcel.

Failure to pay means it will be returned to sender.

I suggest advising the seller that RM says insufficient postage.