1) Setup an external billing product for selling the credits.
2) Add a billing option to the product.
3) Edit the Notify URL for the product putting your URL as shown below:
= Replace # with the amount of credits to add when payment is madeXXXX
= Put your API Key from Advanced Features.yourURL.com
= Put your domain name.
4) You can then get the URL or ~~ code to use when when editing billing for placing the button throughout your site.