You should always make sure you check exactly what you are paying for. GST, shipping, and whatever else. Sometimes companies will also take $1 off your card and then return it, just as a security check.
I wouldn't worry about $0.15.
In New Zealand several years back when our post offices introduced a Visa-based gift card, you could overspend as much as you liked and it'd get billed to the company. They fixed this bug quickly, however it happened.
I imagine if you were to really overspend, the order would have been declined.
Its possible the card has a small overflow allowance. It may be the company that handles the gift cards will later send you a bill for $0.15.
Either way, its no biggy.