Josie I'd use the Live Chat facility and tell them that you were too busy (or whatever) to use the credit within the allocated time .... and that it takes you time to consider the books that you want to buy .... and you want a period of x months to use it.
Well, one can only try
I had a store credit that lasted for ages - I can't remember if they put a time limit or not, but there certainly wasn't one in place.