please advise

Hello all
I'm new to all this and I must admit im finding it rather hard to get to grips with.
I have searched through lots of posts and just cant seem to find one that matches my problems exactly.

I am using the royal mail 1st class standard module and the recorded delivery module for the uk which is fine when I add one item to my cart but I can continue to add further items and the shipping cost doesn t change.

Im using a drop ship company and I cant afford their postal charges to be more than what i charge my customers or i will always be the one out of pocket.

My supplier is using the royal mail by weight and also adds on a fee per item how can i do the same.?

Please explain in terms for a dumbo I have read several replys to posts and needed a uni degree to understand them lol

Thank you in advance for your kind attention
lorraine