I leave an SAE with a 2nd class stamp on it. I'm always owed money at the end of the month but it would be interesting to know if it would be more if I didn't leave an sae. I think it would be - how could it not?
I feel i'm in the customer satisfaction business, rather than window cleaning. I noted that people hate it if I clean when it's raining anything more than light drizzle, and I don't like it if it's pouring (plus I feel a numpty) so I don't bother - I make the tie up at a weekend (but haven't taken a 2 week summer break yet...). Ditto with the sae - it removes a barrier to payment (I offer PayPal and BACS as well as cash too). I'm not the cheapest but my gross margin is healthy. It's worked well for me and i'll continue doing it. I spent £224 in 2010/11 and £108 so far in 2011/12 on postage for the sae's (but had 6 weeks off this year due to broken thumb). I'd say half pay cash on the day, half via post/PayPal.