When a customer wishes to purchase a subscription on our site, they go through the standard WooCommerce add-to-cart and check-out experience. If they already have an account on your site and attempt to open a ticket without a valid subscription or token they will be redirected to the shop page (or any page that you set up in TICKETS->SETTINGS->PAID SUPPORT).
Here are some images from the STANDARD WooCommerce check-out process. This is based on the WordPress 2016 theme. Obviously in your own operations you would likely be using a different theme.