Unfortunately our promo code system requires a payment and we tried to make it very clear on the AppSumo page what the requirements are. We couldn't change our system in time for this promo so we had to run with it as is. If you run into this issue, please email help@twilio.com and we will take care of you.
I knew a payment was required and I'm okay with that. My suggestion is that the payment isn't processed if the code is invalid. This gives the user the chance to correct the code if they made a mistake.