If a builder/glazier wants all the money up front, he is either out to cheat you or he is on the edge of bankruptcy.
We paid someone to do some specialist lime work on the outside of our house, we paid a 10% deposit and the balance, a couple of thousand £s when he was finished. There was another small repair job we wanted done and he asked us to pay upfront with the balance for the main work as he had done such a good job and the extra amount was trivial we paid him - and never saw him again and the small job was never done. We later discovered he had gone bankrupt the following week.
When we employed a builder neighbour to do some work. DH went with him to the Builders Merchant and paid for the materials himself and we just paid him for his labour at the end of each week.
Petition for estranged Grandparents