Hi all, I just bought a pair of $200+ boots using BIN and the seller had listed shipping at $20. I've already paid the seller but she just sent a message about how the shipping charge was incorrect. She's asking for an extra $5 which is fine because it's not much at all. Just for future reference though, if a seller comes back asking for extra charges, do I have to pay it? ie. if the seller asks for $X, could I potentially refuse to pay for the item w/o any implications like negative feedback?