Invoice Template making a term of net 30 constant when creating invoices

Not on the invoice template, but you can select terms for each customer and it will automatically fill in on the invoices.  Edit each customer card.  On the Additional Info Tab, select the Terms for each customer.

Instead of one customer at a time, you may also be able to edit all of your customers at one time.  Click on Lists on the upper task bar.  Then at the very bottom of the list click on Add/Edit Multiple List Entries.  When the window opens, select Customers for List.  Then on the right side of window click on Customize Columns.  Find Terms in the left column and add it to the right hand column (if it isn't already in the right column).  You can move it up to a higher position  -- next to customer name -- to make it easier to see.  Then OK to close columns window.  Now on the Add/Edit Window, on the first Customer select Net 30 for Terms.  Then Right-click on the Net 30 field and choose Copy Down.  This will assign Net 30 to all of your Customers.  If you have Customer:Jobs, it will only assign to Customer level, but will apply to all jobs too.