PayPal rules

I gotta throw in here , there for a short time , paypal was charging me over .03% , but it was under .04% , why , I don`t know unless they were just making mistakes. It happened on 2-3 purchases, but has settled back to .03% since then . ????
In my case, there are usually more trips to USPS than one, to check shipping, then notify the buyer, and waiting on payment thru paypal before I ship. GAS ain`t free here in the rapem area !
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I would think that gas would be part of the shipping charge not 3% paypal service fee.