GET request to payjoin error is common, but handled with a dead end UX

[DanGould]

Looking at payjoin-client logs, lots of people try to visit to the deployed payjoin endpoint with their browser.

They are only met with a 400 error and

{ "errorCode": "missing-header", "message": "Missing header: Content-Type" }

On BTCPayServer, they receive a suitable 405 Method Not Allowed error, but still does not provide an avenue to resolve the problem.

If they're really coming from the browser, it would be better for them to get some help going where they should be going, either by fulfilling the payjoin or learning more about it. @yashrajd proposes this solution:

I suggest one also could display another valid bip21 pj uri without an amount as Call to Action to try again. A redirect to an invoice page may also be appropriate if one exists as in the case of a payment processor.

[Kixunil]

display another valid bip21 pj uri without an amount as Call to Action to try again

Sounds dangerous since some URIs require the amount to be exact and people sending wrong amount causes a lot of pain for merchants. Not to mention bypassing automated systems. You could add an ID to the URI but since address is already unique this increases the length and thus the size of QR code.