Hi @ragnsan
Next.js can still be used as your frontend, fetching product and cart data via Shopify’s Storefront API, Since your client is a Shopify Plus merchant, you can utilize Checkout Extensibility to customize the checkout while keeping it within your main domain.
If you are not using Shopify Oxygen, you can set up a reverse proxy in Next.js , Example: Requests to /checkout on your domain (yourstorename.com/checkout) can be proxied to Shopify’s checkout engine, keeping the URL structure intact.
Shopify’s Multipass authentication allows customers to stay logged in when redirected to checkout, ensuring a smooth user experience.
But yes, your merchant should be on the plus plan for the same.
Hope this help.