Why does my Shopify store say “Checkout is temporary unavailable” to customers?

This error occurs when your store’s primary payment gateway experiences api handshaking errors, when custom shipping zone variables are left unconfigured, or when your store is undergoing background domain verification.

“In my experience, this is almost always a payment gateway handshake failing quietly in the background, or a shipping zone that’s still half-configured. I’ll get to the root of it and have checkout back up within 24 hours for $99.”
— Jeff Bollettino, Managing Director, Ignite Conversions