In principle this seems like a good idea, however, pratically I don't think it'll work.
Firstly, the level of marketing required to;
(a) Get the message across that you are a window cleaner NOT offering a regular but one-off cleaning service. You would probably need to repeat your message 3 or more times, to capture enough peoples attention to get the message across. Therefore, this would be cost prohibitive.
(b) You're then relying on people keeping your number safe thro' the year. There are a very few that will, for example, not everybody can find that chinese takeaway menu when they want it ! And they would probably use the Chinese then than would use a w/c to clean their gutters.
Secondly, you would cannonabilize your existing business; when your current customers recognise that you are allow a percentage of your customer base to have a 'pay-as-you' service, then they will want to follow suit as it would save them a few quid.
In reality, you would probably find four weekly customers slip to six or eight weekly, giving them the best of both worlds ! Whilst the customer wins, you would lose out and I doubt if you would gain enough one off customers to compensate.
Finally, the cost;
(a) You would want to quote a job before you did it, which means 2 visits, two lots of fuel etc,
(B) If you quoted 2 or 3 times the normal amount (as quoted) the potential customer may feel that they aren't getting value for money and decided to postpone or totally cancel the idea of having their windows or gutters cleaned.
Now where I think this idea works is,
(a) In the main the window cleaning fratenity are too willing to dismiss one-off cleans as they would prefer regular repeat customers, unless you're just starting out and any work is better than no work.
(b) The guy walking the streets, is able to both price up & probably complete the job at the same time.
In short, I don't think we should turn down one-off cleans unless we are totally rammed with customers when they come along, but should we try and create, offer and market a one-off clean service as something in addition to our regular service - then NO.