ok, i just got clarification. You have to spend minimum $79 out of pocket, THEN add product to cover your credits.
so basically if you have a $20 credit, then you have to buy $99 worth of live goods. if you have a $100 credit, you'd have to purchase $179 worth of live goods to use the whole credit.
then they add the handling/fuel charges.
as for the question of where to use it, there's a box under the total on the checkout screen to type in how much credit you want to use.