We're often asked how to offer the option for passing donation transaction fees along to the donor. This isn't a feature built-in with Salsa, primarily because we do not actually charge/store the fees. Those are entirely handled by the merchant gateway which processes the transaction.
More on merchant gateways (including some fee information, to the best of our knowledge):
That said, if you know your transaction fees, you could add new amounts to the donation page that represent the donation + relevant fee percentage. Again, this isn't something built-in/calculated by Salsa and would by your responsibility to maintain as a separate donation option.