Log in and under your name select "Account":
On the left, click "Account Credits":
Then you'll see your current credits and a link to view past credits. If you were gifted a credit by Thistle - for the value of a free shot, for example - simply add the shot on your Upcoming Deliveries page here (to the following week). The credits we added will cover the cost of that shot at billing on Thursday 11:59pm prior to the upcoming week.