@Georgia_Peaches is correct in that you pay, say, $200 for the class. Some of the pieces you can make are $200, but if you select a $240 piece to make, you pay the additional $40 at your session.
BUT, on our last sailing, I booked a class for 2 which was offered for $170. I called to book and got another $8.50 discount, so I was billed $161.50. We never had to apply for any discounts onboard and each got a $200 "allowance" for our piece.
Yes, you can book after you board, but I take advantage of pre-cruise sales when possible.....which are not available once onboard.
Call again and ask for your discount now. If the price goes down between now and your sailing, call back, cancel your booking and rebook at the cheaper price.....but either way, you should be able to take advantage of the 5% plus any pre-cruise discounts.
Please come back and let us know what happens!
edit......we did this June of last year. Things might have changed in the last year, so YMMV.