Shopify Checkout APIs down
Incident Report for Upscribe
For a 30 minute period we observed an increased error rate in responses from the Shopify API, specifically relating to checkout operations. Shopify has not provided any status update or explanation for the outage. Upscribe checkout was degraded during this period.

Many Twitter users have also observed the outage:

Our internal monitoring shows the increased HTTP 500 "Internal Server Error" responses from the Shopify API:
Posted Sep 09, 2021 - 09:00 PDT