1. Forward a fake card to Shopify. We sent a request to Braintree to ask Braintree to forward a card vaulted in Braintree to Shopify. The request from Braintree to Shopify is still hitting https://elb.deposit.shopifycs.com/sessions. The card we vaulted in Braintree before and used to forward was still the fake one 4242424242424242.
This request was always succeeded and we successfully got a session_id.
2. It's exactly the same as the step 2 in the above success case. However, we got a response with a 500 error, but no more details.
Comparing these two cases, the payment transactions both happened on a Shopify sandbox side. I don't get why a braintree fake card number is related or why it's supposed to work in Shopify sandbox but the 4242 one won't (we also tried a test card provided by Braintree but it doesn't work either). I suspect it's not about the card number. There might be some other issues that we have no clue. We will really appreciate it if someone in Shopify can take a look at the exact logs of the failed requests, and understand why it's a 500 error. In general, even if the session_id may not be valid, we should expect some 400 errors (invalid request.)