Hi!
The button (Buy Credits) in the ArcGIS Online for Personal Use has link to the renewal of subscription page instead to the buying additional credits page.
There is not any option in this page to buy additional credits. In order to buy additional credits the customer has to refer to the below link:
I think there should be two buttons in ArcGIS Online for Personal Use;
1. For Renewal of the subscription
2. For buying additional credits. ( There should be option for choice to buy Credits, now there is only one option to buy block 1000 credits.)
Best,
Faiez