SliQ Invoicing Plus allows you to define a set of payment terms on its Setup/ Payment Terms tab as described in this help topic:
For example, you can edit the terms to include a 45 day payment term.
If you want your new payment term to be the default added to an invoice for a specific customer, go to the Customers & Statements tab and select the customer. Then choose the correct payment term in the Standard Terms field in the Customer Details section at the bottom of the Customers & Statement tab.
A similar process can be followed to set a default payment term for a supplier - go to the Suppliers tab and choose the correct payment term in the Suppler Details section.