The Order Exchange will take invoice terms directly from the customer in Xero, if these are not set it will use the terms of payment set up in XERO for all clients and if no terms have been set for Payment Date The Order Exchange will default to 30 days after the invoice date.

Please refer to XERO's help guide for more information on payment terms:

https://central.xero.com/s/article/Set-due-dates-and-payment-terms-for-invoices-quotes