Ask HN: Simplification of multiple payment processors?

What's a good approach to handling support for multiple payment processors, with minimal tinkering?

Are there any existing libraries or services that abstract out the many differences between payment processors? I'm looking to write minimal server-side code, if any. Turn-key solutions (i.e. the sort of thing Stripe.js provides, but for other processors) would be ideal.

It doesn't need to be free, but free is good.

All I would need is for any payment completion to trigger a callback URL, and my server would handle the rest. I don't need subscriptions or anything fancy, really.

This must be a very common problem, but I can't really find anything that hits all these sweet spots.

Hell, even a simple HTML/JS generator that wires most stuff up for you would be nice.