Little bit of help required with setting shipping rates please

Does anyone know how to solve the following scenario with fairer shipping rates?

I have two products and both weigh 250g. One is slim enough to be classified as a Royal Mail large letter which costs £1.40 to send.The other has to be a RM small parcel which costs £2.95 to send.

Is there a way to set the correct shipping cost for each on the amazon system?

Thank you kindly for any replies.