Going infinite on gift cards

Shaw’s, among many other retailers, is offering a promotion where one can buy a $330 gift card at a cost of $300, or a multiple of those numbers, any number of times, the theory being that government economic stimulus payments are often in multiples of $300, and they’d rather you spend it at their store.

I suspect that their POS system would allow one to pay for a gift card using another gift card. Gift card is a method of payment, and the cashier likely doesn’t even see what kind of card you’re swiping to make payment. (They just enter in a $330 gift card activation with a $30 store discount.)

So, I think that it would be easy for one to buy a $330 card for $300, and then on one’s next trip (or right away if one thinks one can pass it by the customer service rep.), use $300 from that gift card to get another $330 card, meaning that you now have a $30 card and a $330 card, for $360 in total gift cards for a purchase of $300. This process should be repeatable as often as you think you can do it without them throwing you out of the store, leaving you with n $30 cards and 1 $330 card, all from an initial $300 expense.

Not only that, but since in Massachusetts the law says they must refund the balance of a gift card in cash if it’s at least 90% used, you should be able to convert all those $30 cards back into cash, more than covering your initial $300 expense.

Is there a flaw in my logic? Is their computer, customer service rep., or manager likely to notice? Is this doable, legal, moral, and/or ethical?