Yeah don't keep trying it. They are billing you every time you try it.
I had no problems the first time I used them. The second time I used them I did this, twice, and got billed twice. I requested a refund and got it as a credit. Fortunately they refunded me the full price and not the discounted price they charged me for the special.
Later I tried it again, and same thing. I did not even bother asking for a refund, because their system is too dumb. Why do I want another credit that I can't actually use?