Bear in mind I haven't tried this yet, but I love euchre so I probably will...
1) I will not buy it if it has a nag to donate more after I already have. I'd say just ask for a fair price and leave it at that - pay what you want is fine too though... I just don't want to see a donate button on a piece of software I've already purchased.
2) You said you'd like to add multiplayer, that's cool, could you add local multiplayer, then you wouldn't need matchmaking services?
Thanks for the game!