Podpage isn't an ecommerce platform, but it integrates with all of them the same way: you just need a link to your product or store. Add those links to your Podpage Storefront and your products appear on your website.
For example, you could self-publish a book, sell it on Amazon, and use the Amazon link as a product in your Storefront.
Where to find your product link
Every ecommerce platform gives each product a shareable link. Here are some popular examples:
Thrivecart
Thrivecart is popular because it offers a one-time purchase. When you set up a product, Thrivecart shows a link for it.
Gumroad
In Gumroad, the link to each product appears under the product's name.
Payhip
Payhip is free to use (they take a small portion of each sale). Set up a product and click its share/embed button to get the link.
Pensight
Pensight has free and paid plans. Click the ... to the right of your product and choose Copy Link.
Many other options
Samcart, Shopify, Etsy, and others work the same way. If you can't find your product link, check your platform's help section or contact their support.
Tip: If you run a complete store on a platform like Shopify, you can also add the store's link to your navigation bar instead of listing individual products.