Hey Matteo,
what I suggested is my hunch of what could work or what I would look into. In more detail you would need to keep track in javascript of the current user font selection. On the plugin repo there is documentation of the javascript events it fires. You probably want to get the current font on “fontsampler.event.afterinit” and then update it whenever “fontsampler.event.activatefont” fires.
WooCommerce has a couple of javascript events also, you probably want to use the “adding_to_cart” or “add_to_cart” to listen for when a user adds something to the cart. I don’t know if you cant pass in product meta data in that event directly, or if you need to send another AJAX request to your theme with the product id the user added, and specifically send the font name and on the backend of the AJAX call add the meta info to the customer’s cart item. Here is a tutorial with some different approaches to adding the meta data, but the essential bit is this method by which you can add any kind of meta data to a WC_OrderItem — if the meta entry does not start with a _ it will automatically be added to the order confirmation, order backend etc.
Again, above is not a concrete instruction, but how I would start to go about it ??
