As an Audible veteran of over a decade, my advice to you would be:

Forget the canceling and re-enlisting to obtain a trial month. This has a slight, though not inconceivable, chance of backfiring. I'd suggest switching to the $14.95 single credit now, leaving you with either six cents "lost" credit, or a buck of tax to pay.

I find two credits per month, resulting in two books @ $3 each (depending on tax) out-of-pocket a terrific deal, but there's nothing else I want to spend this Amex credit on.