Audible seems to hide this
link for purchasing extra credits at a discount if you have no more than one credit in your account. The discount depends on which membership plan you have; I have the basic one-credit-a-month plan and the extra three credits cost just under $36.
If you just got the $10 coupon for using three credits, the coupon balance is automatically applied to the purchase, which means the three credits effectively cost under $9 apiece.
For me, this worked out better than trying to find inexpensive books to use up the coupons.