How to troubleshoot payment issues when Shopify checkout fails

When a Shopify checkout fails, you can't just ask the customer to try a different card. Most customers won't come back to tell you why the payment didn't go through; the ad spend is already gone, and the order is stuck at the final step. When I troubleshoot, I first go through the entire process myself on a mobile phone to see if there are any conflicts with the country, currency, shipping fees…