Amazon fees questionable

I sell all the same items, I sell one for £4.41 and get charged 0.65p I sell another for £4.20 and get charged 0.74p, but yet when I list the items it states 0.49p, remember these are the same item, but charms are optional, hence 0.21 difference in price. I sell another one for £4.20 and get charged 0.61, surely the charge should be more on the one that adds the charm, and the 2 at £4.20 should have the exact same charge of listed fee of 0.49p. I then sell another for £4.41 and get charged differently from the 1st one at 0.61p. Mmmm are we being taken for a ride here.