I'll second 2checkout.com $49 to set up and thats it, no monthly charges just a % of the overall product price (around 4-5% last time I checked and 30c per transaction so selling something at $20 would cost you $1.30.

If you are dealing in low cost - high volume then 2checkout is great but if you have high cost - low volume the the 4-5% may not be worth it and the monthly cost for authorize.net may well outweigh the charges from 2checkout.

Depends on what you charge and the volume you shift as to what is most viable to you.