I have no clue how to make it apply. Everytime someone mentions that they were able to, I give it a shot. It's supposed to cone off by default, right? If you have credit (GC) and promo, which gets taken first?
I didn't know about this until it was mentioned quite recently or I would have done so and not been muddling about without it!
Kurish, here is a guide that my help you and others. By default the payment method is set to your primary credit card but you can change that to gift card:
Use this link to gift Amazon Prime (to anyone including yourself)
Gift Amazon Prime
For recipient's e-mail enter yours and click on "save gift options". You can schedule delivery date a year from the day of (in my case the day mine expires).
It has automatically selected my credit card for method of payment. Click "Change" next to "Payment method" to change method of payment:
Select gift card balance from here and click continue:
Now you will see gift card is the method of payment and under order summary you will see your order total $0.00 (if you have funds to cover the order). Click to place your order.
Congratulations, you just gifted yourself Amazon Prime.