In-between decisions is a stressful place. You're either going to do it, or you're not. Make your mind up quickly, do whatever needs to be done and the stress will go. But many of us come to window cleaning because we want a less stressful way of life, so why add it for the sake of £14?
Just let it go, in your head too, shrug shoulders, and focus on your good customers. This will no doubt happen again at some stage; it's pointless getting annoyed when it does. It's just part of the job.
And if you chat with some builders, they'll be to give you some proper horror stories where they've purchased materials, paid for staff, and then been done over. We have it easy sometimes (not the guys with huge accounts 'n' staff).
And it really isn't personal; they'd have done it to me too.