Online membership donations – an obvious solution I'm probably missing

I work for a small nonprofit, and we’re looking for a membership system/plug-in that allows individuals to ‘donate’ but doesn’t heavily interweave it within Registration. To this point, I’ve checked into Paid Membership Pro, but it seems to hijack the entire reg system.

In my ideal world: users can register freely on the site without any interruption, but can use their unique information to choose a donation membership ‘Level’ if they so desire.

Any thoughts? Suggestions?

Thanks!