It's important to understand that Pocket Prep is what's known as a third-party developer. We don't have access to your personal or payment information when buying through any of the major app stores. While that's is a good thing, it prohibits us from helping with payment-related issues.
Payment help for iPhone and iPad users
Since all iOS-based payments are made through Apple's system, they are the best resource to help you sort out any errors you may be experiencing when upgrading the app to premium. Check out this iTunes support article which suggests fixes for common payment issues and errors (see Billing section towards the bottom right).
You can also reach out to an iTunes support representative for more personalized assistance (see Apps and Software section).
Payment help for Android users
Google processes all payments and handles all financial activity on our behalf, much like Apple does for our iOS users. Since Google's the only one who can help you with your actual Google Play account, they're your best choice for payment support. Check out this Google support article which suggests fixes for common payment issues and errors. You can also reach out to a Google Play support representative for more personalized assistance.
Payment help for Web-based users
Stripe is our payment processor of choice. If your payment information isn't correct or your credit card has been declined, the app will let you know what needs to be fixed. Try using a different a credit card to make the purchase. If you continue to have issues making a purchase, contact our support team and we'll work with you to understand what may be causing the problem.