Checkout dynadot, deposit more than $500 and you get bulk pricing ($7.50 for .com) and API access. The good thing about their api is that it's a simple request: (http://api.dynadot.com/?register=domain.com) so you can whip something up in php within minutes.
If you own more than 20 domains on namecheap (or 200$ credit on your account), you can hook a number of API's up to the account such as from scripts like WHMCS, though I'm sure the same concept can be applied to an auto-registration script.
Internet.bs has a public API and you can get .com for $6.95, and that is without the bulk purchase option, Contact them for a bulk order they usually have something better if you can use mass paypal payment etc.