My two cents...
1) Don't credit up a machine beyond the multi-game discount settings (3 for $2.00, etc). 8 credits is over the top.
2) If the person refuses to play doubles, and you've been playing for that long already, offer to let him/her buy your credits off of you.
I think offering to let the guy play doubles with you was a nice start, but alas, not everyone wants to play doubles.