In Xero payment terms is only setup with a DueDate. This means that things such as early payment discounts cannot be natively supported.

The due date for Sales invoice inside Xero is not the Shipping Due Date. It is the "Payment Due Date".

The due date is calculated based on the customer settings inside Xero. The required number of dates that needs to be added to the order date is specified in customer settings.

Based on that it adds that number of dates ahead to the order date when calculating the Due date.

Note it is possible to write the payment terms into the comments field by setting the comment to {PaymenetTerms.Description.Value}