In other words, 50% of each purchase must be cash, the rest you can use your credit.

For example:

You bring in a book with a cover price of $7.99
Annie's give you a store credit for $1.35
You choose a pre-read book from Annie's with a cover price of $7.99 = Annie's Price $3.99
You use your store credit of $1.35 and pay only $2.64!