My method of payment didn’t work

I have a question regarding in-app purchases. I just got a $20 gift card for the iTunes/App Store and put it on my iPad. After that I went to purchase a certain pack in an app that I already had called “launchpad”. The purchase was going fine. It cost $1.49 exactly and it did the same thing where I have to enter my password to confirm purchase. Once I did that though, for some reason it asked me to continue to the App Store because I needed to sort out the ‘method of payment’. Thought it was all normal, so I went in and saw my account which had $20 dollars in it, pressed on that method of payment...and it didn’t work. It didn’t shut me out, but there is just no way I can confirm the purchase. No money has been taken out either. I just want to know how I purchase my item so I can get on with it. Thanks.

[Re-Titled by Moderator]