We have a few vendors who like to use very long invoice numbers. So long to the point that there are too many characters for the document field in AP when I am entering the invoice.
That field will hold 12 characters. Is there a way to increase this limit to 15 or 20 characters?

No, use the last 12 characters. The odds of Softrak increasing the length of that field is close to zero.

If you want the vendor to see the full invoice number when you pay them, enter it in the Description field (#1 or #2) and whichever one you decide, print that field on the check advice.

Steve