Im a mix of both of the above.
I posted a letter to each of my customers after their cleans to let them know about this new payment option. I offered a 50% reduction on their next clean for doing so.
However all new customers must sign up before I clean their house. For example I quoted a job the other day who has accepted the quote n details but I wont be cleaning their windows till next month. 2 days after the quote, they have signed up well in advance. So far no new customer has battered an eye lid at having to do so. In fact about 80% have commented how good an idea it is or that its more professional. One customer has recommended me to their friend, just for this reason alone