It depends on the individual store:
With most stores, you may earn credits based on your net purchase (subtotal) amount
With some stores, you may earn a flat number of credits irrespective of the purchase amount
With some, you may earn a little bit of both!
With some stores, you may earn credits only for items in certain categories. See the store’s Special Terms.
Net purchase amount excludes taxes, shipping, and handling, fees, gift-wrapping, coupons/discounts or credits, returns or cancellations.