How to buy additional images? (Optimize Image Tool)

I bought the paid version of the WP Fastest Premium plugin.
I have over 1000 images on my site.
How to buy additional images for optimization?
The function (Choose the amount of Image Credit you’d like to add.) has an error.
https://api.wpfastestcache.net/paypal/buyimagecredit/
The page is not available.

The page I need help with: [log in to see the link]