Double paying for shipping

I hope someone can shed some light on this issue and how to solve the problem. I’m FBM seller, I sell one item and it weighs 7.5 ounces, I always use Amazon Buy Shipping . I linked my FedEx account to my buy shipping account like requested. Sometimes when I select FedEx 2 day one rate( 12.95) I also get debited from my checking account. The amounts have been ridiculous. $57 $49 $37. So I’m double paying and it eats up all of my profits and then some . I’ve emailed FedEx with copies of the debits. The crazy thing NONE of the numbers correspond to any numbers with Amazon or any of my shipments. I can’t cross reference any numbers . Anyone got a clue how I proceed? Thanks