I'm going to find a loophole in a stores promotion to get free coffee
Is there a reason you are trying to abuse a loophole?
Yes, I have always wanted to do that. It means I have found something out that I have always wanted to do.
Using a loophole to get something free is no different from theft, it is unacceptable. You should be honest and purchase the product if you want it.
I don't plan to get the coffee, I just like looking for and finding loopholes.
Well, as long as you are not taking undue advantage of this store, then that sounds acceptable to me.  It can be a creative hobby to try to decipher rules and find unintended loopholes.  Perhaps the best thing to do now is inform the store of what you have found, and they can prevent losing a lot of money (and maybe even having to fire employees) as a result.