Are you running these transactions in live or test mode?
Are you using a valid credit card with the correct billing information?
AVS mismatch means the billing info being used on the form doesn\'t match what the credit card billing info on record is... the address, the ZIP, etc. There ARE settings for this in the Authorize.net account that you may want to play with.
Also, if you were using a real card, try a different card and see what happens. Just some items to check or try.