Further to that, TxForge is a tx builder that provides an estimate fee function that is implemented the same was as in Minercraft. It defaults to 0.5 sats per byte for both data and standard rates, but you can provide a rates object from minercraft to calculate using different rates.
-> https://github.com/libitx/txforge
@exist right now only USD. To clarify, invoices are always raised in satoshis. When rendered the widget displays the USD amount alongside the satoshi amount.
I could make that an option you pass in when you embed the widget - eg you tell paypresto the currency your users wants to see.