Simple Domain registration APIs?

Hi all,

I am currently creating a site where, as part of it, I want a user to be able to choose and register a domain name. So far, the only way I've found to implement this is to sign up for one of various reseller accounts and they all want several hundred dollars of deposit or other payments. They all seemed to be geared towards hosting resellers too.

Does anyone know a really simple service I can use where I sign up and get charged on a pay-as-you-go basis?

Thanks in advance.