Hi Paul
I clean a rather large showroom each month, inside & out. Exterior is WFP, interior is Trad, I use a Wagtail on a pole for the high level panes (top is around 24'), I also do the internal offices (all glass floor to ceiling) every other month.
Quite straight forward stuff, however wasn't sure on price (each pane is round 15' x 8') so based it on estimated time+ and asked client what they were willing to pay (rather unusual step), turned out to be around the price I had in mind so got the job!
I insist on prompt payment (normally within 2 weeks). I would however advise you don't put all your eggs in one basket i.e. become beholden to one client, nice whilst you have them but a blow when they go!
Hope it helps
John