You can set the due date through the invoice in Buildxact and this will be picked up in your accounting package.
Create the invoice/receipt
Enter the due date in the "Invoice/Receipt date" field of the Invoice
This date syncs through to your accounting package, and applies this as a payment term for the contact (Supplier or Contractor) associated with the the invoice.
Within your accounting package you can set the payment terms as a system wide default, or per contact, or both.