Different Methods To Automatically Apply An Account Credit

What are the different ways to automatically apply an account credit with a purchase?

I know I can add a credit to a collection so it is automatically added to the client's account when it has been purchased. Are there any other ways? Can a credit be added to any other photo products, price lists, or store items?

The only other way us to include a credit with store items.

Tim, thanks!

I tested out automatically applying a credit with a Collection. Is there a way to populate the credit description information so I can see what the credit was from when I view a client's account. The field is currently blank.

The credit in collections take off from the a la cart items in the cart. The credit doesn't go into someone's account. 

The credit from a purchased store items shows the order number.

I tested it out and it let me purchase a service with the credit that was applied from purchase a collection (w/ credit associated to it). This is what I want to be able to do! The credit appeared in the "My Account" area too. 

Specifically, what won't it let someone purchase with a credit that was applied from a Collection purchase?