Once all the paid image optimization credits are used, whether they are one-time credits or part of the 500GB included in the Unlimited plan, both Autoptimize and ShortPixel Adaptive Images will serve the original images. That means your website, instead of displaying optimized images from our CDN, will display your unoptimized images. Please note that the images that are already in the CDN will be served for up to 30 days.
If you are in need of purchasing extra credits, feel free to do it from your account. You can also check out the unlimited and the one-time plans here and here.