Hi there,
I would not pay a cent without having a Building Agreement in place. I would insert every detail about the job into this agreement i.e. completion date, price (whether this includes VAT) and all the details of the job to include for example..... Supply and fit 3 teak doors.
Supply and fit architraves( clients selection), paint the doors with three coats of paint..... I would detail everything that you can think of. In the event of a dispute, you will have an agreement which you have both signed to rely on.
Good Luck