Forms which collect either a user defined or user selects payment amount are often referred to as "donation forms". These payment/donation forms usually have some pre-defined dollar values ($1, $5, $10, $20) in addition to an input box for the user to type in whatever number they want to pay. After selection, the user can choose a payment method to "check out" and make the donation. Authorize.net is a very popular credit card gateway. This document is intended to outline the options available for donation forms inside of drupal with authorize.net.

"Donate" (https://drupal.org/project/donate)

Payment For Drupal Commerce (https://drupal.org/project/payment_commerce)

"Payment Donation" (https://drupal.org/project/payment_donation)