They get run as a credit card, so as long as your BB&B accepts credit for the GCs you should be good. And yep, math is correct
The downside is only $50 Happy cards so you need 10 and have to wait for shipping. But if you can't do better than 16.66% in the time you have left, this seems easy.
Do you have a store in the Safeway network nearby? You could get one at 10% discount to try it before putting the large order in. I'm pretty confident in it, so I'd risk it, but I don't have first hand experience to give you on this one yet.