To be able to look up Bank holidays properly you will need to set up a holidays table that has the bank holidays listed specific to the country the database is being used in. You can then check if the date is either a bank holiday or a weekend using a query.
If it's multi country things will get complicated.
assuming you have the same table and field names as identified in the code, put the function in a module (not a form module) and call it - the value for duedate would be date() and transit days would be 1