The main reason for a store to sign up on a website would be:

• Advertising
• Centralised shipping
• Centralised handling of payments (and note, this one is especially hard due to laws surrounding KYC and complexities in handling different payment methods)

The Fediverse, being decentralised, has a hard time implementing the latter two. The first is basically not much different than being discoverable on Google.

So fun as it sounds, it won’t be easy to implement. You’d likely have to have independent “shippers” and PSPs sign up to this, and somehow have webshops choose which to use. And that’s a very awkward structure for a Fediverse-minded solution.