You are generally correct that this kind of adjustment is considered a non-taxable discount or price adjustment. The only time it would be taxable is if you get more back than you paid. For example, if you paid $300, get a refund and keep the product, that is not taxable because the company is allowed to give you the product for free and that does not become taxable income to you. However, if you ended up with a free product, your $300 back, and another $100 gift card, the $100 gift card is technically taxable income (you got back more than you paid).
