From Ammoland:

The vending machines in question, from American Rounds, use computer technology to validate age with picture ID. If the person attempting to purchase ammunition is over 21 years old, they may purchase the ammunition using a picture ID, which the machine validates using facial recognition software. Only credit or debit card purchases seem possible. There does not appear to be any way to pay with cash on the face of the machine. This means a double check of identification takes place, as the person whose identification is validated must also use a credit card, which has to be separately validated.