Are you looking to make money, or looking to make the process less of a PITA for your customers? Based on your mention of API, I'm thinking more the latter, but just want to make sure.
I have not yet used it, and I'm kinda dubious about any "requirements" they might have for API access, but
Namecheap has an API and
says they don't charge extra for it.
As an ICANN Accredited Registrar, you can rest assured that they will have at least the minimum required quantity of lying sacks of crap and greedy douchebags, but they might be over in the web hosting department instead.
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