Log into OnFrontiers at https://onfrontiers.com/login

Go to the Team tab > and click the gear logo for Team Settings

Click the Credits section on the left of the page

Select the number of credits, see discount & total amount.  Here's a reference on how many credits to buy.

Click Purchase

If no card is available on file, you'll be prompted to add one

We'll promptly charge your card or bank, load the credits to your account, and email you a receipt