Integrating lesser known e-payment gateways full of surprises?

I've integrated about half a dozen e-payment gateways, from popular ones (like paypal) to not-so popular ones.

It seems every time I try to integrate a non-popular epayment gateway, I exceed my original time-estimation.

I think I'm a decent programmer, but my time estimations probably needs work.

Do other programmers run into many "surprises" when integrating e-payment gateways they've never heard of?

Any advice will help.

thanks

Yes this is totally true. Payment gateways are very finicky and even the big ones like have strange quirks.

What I find is the main issue is documentation. The more well known gateways have reams of good documentation. The lesser known ones have little bad and out of date information.

This isn't limited to payment gateways. I've encoutered the same thing on IVR's, Fax Services, (The USPS api is an example of a very good one ). It doesn't matter if it is soap, rest or simple query strings, if the documentation is bad you are forced to guess and check. This debugging takes forever.