There are two ways to do a trade in. If you select trade in when you order your new device, then you should get a refund on whatever payment method you used when you ordered the device. Although if you did carrier financing, I’m not quite sure how that would work. You can also just do a stand alone trade in that’s not attached to a device purchase. The stand alone trade in will result in a gift card. It sounds like for some reason your trade was processed as a stand alone trade in instead of with your device purchase. It’s probably worth calling apple to see if they can do anything, but at this point it might be too late to change it.