#Monerujo is not just an #OpenSource wallet for the only private-by-default cryptocurrency out there, #XMR / #Monero, it's also a damn good payment app generally. My only complaint is that, because it doesn't rely on an external service to do the heavy lifting, it is a bit slow, especially over Tor.
It even integrates xmr.to to enable #Bitcoin payments, though one might not want to hold a large balance in XMR since it doesn't appreciate the way BTC does. No way to receive BTC yet, though.