Sign up with no price What we need is essentially a free product order. We need viewers to be able to sign up for free classes where there is a limit on the number of students. The registration requirement and the inventory limit indicate that it should be a shopping cart function, but the module that allows for free orders is only available in a Drupal 6 module. Is there such a function for Drupal 7?
Ubercart for D7 has a built in free order payment method so the UC Free Order module isn't needed here. You can configure the conditions under which the payment method is available under the payment method configuration page (Store > Configuration > Payment method).

If you need to limit available purchases, the included UC stock module enables restricting products to a limited quantity of available purchases.