Morals or not, the business has every right to that $35. You were credited $35 by mistake, and you knew it the second you placed the order. You didn't question the fact that your $40 gift certificate took $75 off?
If a banker adds an extra zero by mistake and your $1,000 deposit turns into a $10,000, you really think you're entitled to that extra $9,000? Not in this life buddy.
It's an obvious mistake on AHS's part and the correction should be made. You didn't think they would catch it but they did. Just like if you spent that $9,000 extra dollars, YOU would be held responsible for it and would be required to pay it back. Absolutely no different in the least.
All too often people play the "the customer is always right" card. The customer is not always right. Austin Homebrew is a fantastic company and I'd be interested in seeing what they do here with a stubborn customer.
If I were AHS, I'd look at my options... it'll be too much of a hassle to file a small claims suit against you for $35, but you can absolutely take your business elsewhere. In fact, if it were me, I'd probably make sure you weren't allowed to place an order at AHS again. I'd also let others in the industry know exactly what happened, how you've taken advantage of a mistake illegally, and let them decide what to do with the information. I certainly would not want people like you shopping at my establishment, however.