Being a full time postie does have its benifits. You get paid holidays, paid sick and the provision of a pension whatever that ends up to be. We don't get that. Self employed is not considered as permanent employment until you have been at it for a number of years. Loans are harder to get as the criteria is different.
When we had the bad weather a couple of winters ago, we never worked for 5 to 6 weeks. No work = no pay. You have to be very disciplined to put money aside for a 'rainy day.' I know of a roofer that only did 3 days work during that period - he applied for a post as an additional postie over the Christmas period and has been there ever since. This is his first employed job and he loves the security of it. He now does a bit of roofing in his spare time. Sometimes the grass appears greener on the other side, but it isn't.
Most jobs today are boring and repetitive - window cleaning is no different IMHO. Try to identify what the real reason is why you want a change of career. Most posties were very upset when the post office changed their terms and conditions and working hours to reduce costs. Our postie who has been on the same job all her life doesn't work 6 days a week, although a Saturday is one she does work, but does get one off in the week.
The job is getting more and more regimented, but things aren't easy anywhere.
Working for yourself isn't for everyone. Some of us were 'forced' into this status as I was too, some of us became window cleaners after leaving school, and some have seen this as a business opportunity after a few years experience in the industrial sector. Some have made a thriving business from it, but there are others , many others, that haven't made the grade - they were sold or sold themselves the dream which they didn't/couldn't achieve and have moved on.
As a window cleaner, you have a stigma cast upon you by virtually every other occupation. You are the lowest of the low in most peoples social pecking order.
Stay with the day job as has been advised, canvass and clean windows and build up your business in your spare time. This is going to mean sacrifice, not only yours but the whole family's.