I recommend not listening to most of the suggestions here that say build out some stack -- that will be a distraction and a lot of time to spend before even get

I recommend not listening to most of the suggestions here that say build out some stack -- that will be a distraction and a lot of time to spend before even getting your first customer. The most likely outcome of any startup/project is that it will get ignored and you won't have any customers, so don't spend a ton of effort building out custom user/payment stuff. Since you already have a WordPress site, add WooCommerce (https://wordpress.org/plugins/woocommerce/ https://wordpress.org/plugins/woocommerce/). It's a general-purpose eCommerce solution and has product pages, user registration and login, accept payments, etc. The one bit it doesn't do out of the box is recurring monthly paid subscriptions, for which you'll need the WooCommerce Subscriptions add-on. If your thing takes off, either this will all be "good enough", or it'll make sense to build out something custom.