If you're used to sending orders to Xero when they're created, especially in a B2B context, you may find it tedious to manually adjust the due date of these invoices (i.e. to the 21st of the month).
There's an option to follow the Shopify order's payment terms and attach the Xero invoice's due date to match that of the order you create or receive on Shopify.
You can access this new setting in the Order Settings.
Please note this option will only be available if you've configured your settings to send invoices to Xero when they are created.