I also bought those I was missing:
I bought 9 books for a grand total of
$17.78 ($1.90 x 7 + $2.24 x 2). I was able to use the discover15 code on all except two books (
My Lord John and
The Spanish Bride).
For some reason my credit card was giving problems so I bought a gift certificate with my Paypal account and then bought the books. It was a hassle but I seem to have avoided the taxes in this roundabout route *shrug*