Currently the page title of pretty much every page is Shopping Cart (or whatever the sites equivalent is), this makes SEO almost impossible as well as making bookmarked pages very unhelpful for end users.
This also relates to this in-progress requesthttps://requests.whmcs.com/topic/products-links-seo-friendly-urls
There needs to be uniqueness for each page and per language including across languages for SEO to be successful
As a minimum the title should be definable on the category however I would like it to go much further.
As an example
Currently you would create a category for SSL certificates and add a number of products to the page with a practical limit on how much content can be included and readability maintained
The category page will be indexed (with a title of shopping cart etc) and the generalised SSL content indexed - or potentially ignored for spamming due to the amount of repetition.
Someone searching for a specific such as a EV Validated SSL Certificate will be most unlikely to see any WHMCS sites in the results over other vendors/presentations methods.
Instead a category page should be a summary page with option for add to cart but also an option for details and for an individual product to be selected. This would then result in a specific page for the product, with optimised content, to be indexed, the URL format for these pages should follow the friendly URL format including for the product itself rather than PID=xxx so you would gethttps://www.my.site/en-GB/ssl-certificates/ev-validated-ssl-certificate.php
which would be properly indexed in English while https://www.my.site/fr-FR/certificat-ssl/ssl-certificate-valide-ev.php
would be properly indexed in French