If you are receiving the error message "QuickBooks Error: Invalid Customer" when you attempt to save a Checkout transaction, this may be because the client record is inactive in QuickBooks Online.
To fix this:
1. Close out of the Checkout screen.
2. Go to QuickBooks Online and set the client record to active.
3. Return to Full Slate, go to Setup > QuickBooks Sync and manually refresh the sync.
4. Start the Checkout process again under the Schedule tab.