I'm looking for a very simple billing script that allows the customer to enter only very basic billing info and address, and the amount.

Once submitted, it will bill them that amount. I use Authorize.Net.

It's basically like a "virtual terminal" that my clients can use to bill themselves whenever they need to give me money.

Let me know if anyone knows of anything like it. It's very basic.

You could copy & paste some info right from the Authorize.Net SIM documentation.

For instance, load up this PDF doc:
http://www.authorize.net/support/SIM_guide.pdf

Then go to page 12 and you'll find a form virtually "ready to go" with minimal customizations needed.

Hope that info is helpful! :)

Also,

Not sure if this helps or not. But here are some prebuilt scripts that you might be able to hack around a bit. At least the coding part would be done.

http://www.authnetscripts.com/

Dunno, good luck!