For your European customers you should likely make it more clear what stripe.js does before urging them to install it on every page of their website. Using it as soon as a user has a probable interest to purchase a product (e.g. when he/she clicks on “Register” and chooses a plan) would very likely be acceptable as a legitimate interest under GDPR, tracking all users even if they don’t have a clear intent of purchasing something and e.g. only want to get information about the product will definitely not be acceptable as a legitimate interest and would therefore require clear consent first.