Payment Services Exception when trying to create an Apple Pay Session

I'm trying to create an Apple Pay session for my website. I'm starting with curl for now, just to get proof of concept.

curl --cert cert.pem --pass {passphrase} --header "Content-Type: application/json" --request POST --data '{"merchantIdentifier":"{merchantIdentifier}","displayName":"testDisplayName", "initiative": "web", "initiativeContext": "{domain}"}' https://apple-pay-gateway.apple.com/paymentservices/paymentSession

This is the response I get back

{
    "statusMessage": "Payment Services Exception merchantId={VERY-LONG-ID} not registered for domain={domain}",
    "statusCode": "400"
}

I'm not sure why this is happening. {domain} is in the form of sub.site.tld with no protocol, such as https, which matches what I see in the list of the domains in the merchant identity dashboard. The {merchantIdentifier} also matches what I see in the top right, but the merchantId in the response is something I don't recognize. It's a long string of characters that appears to be hexadecimal.

I added the apple-developer-merchantid-domain-association file to my .well-known directory and the dashboard does report that the domain is verified.

I am making the request from the web server that the domain resolves to, if that matters.

I can't think of any reason this would be happening.

I'm not sure where the long merchantId in the response is coming from. Does it matter that it doesn't match what I supplied in the request? As far as I can tell, I am using the correct merchantIdentifier. It matches the dashboard and the CN field of the certificate.

I found this other post that seems to have a similar error: https://forums.developer.apple.com/forums/thread/671227

The main difference is a 417 status code instead of the 400 I got. But the problem here was that there was no payment processing certificate and I do have one of those.

I haven't checked with my processor to verify that the certificate is published, but I will do that soon. I wouldn't expect that to matter. Maybe it does?

What other reason could I be getting this error?

Could it be a problem with my merchant identity certificate? It took a lot of effort to make it work. But I suspect it's fine, otherwise I wouldn't be getting a response from Apple at all. I can't think of any other possible problems.