Change how fee's are charged to avoid refunding

[MauroToscano]

• Refunding via tx.gasprice allows a malicious user to provide a huge gas price value, and that could drain the contract. This is particularly impactful if the user is also the block producer of this transaction, so they’d effectively pay themselves (as only the base fee is burnt) and steal the funds from the contract via the refund based on tx.gasprice