If you are willing to give it a go as its glass , fair play to you .
But to be safe you should make them aware that there is a possibilty of scratches , use a nice slippy cleaner and get plenty of good blades , and dont forget to price them in to the cost.
When my brother first started up he took on a travel lodge builder clean , he had to spend loads on blades as it was so bad with render , and he hadnt factored on using so many it ate right into his profit .