It aint what you say its the way how you say it.
Go to see new customer, have a look at windows, admire garden, pat dog, smile at kids. Then ask how often she would like to have then done. DO NOT mention money.
Explain that its entirely up to her, you set her up on the computer and whatever she says thats how often her name pops up. But try to make her feel that its her decision. Thats important
By the time thats done she has already more or less committed so then you say it will be £15, thats our standard charge, like you have assumed its a done deal. Then go straight into I can fit you in tomorrow, would the morning be OK? And make sure you clean them as soon as poss after you have been to quote.
Don't say I'm going to charge you extra (FFS!)
Then after you have gone she thinks. £15 seems a bit steep but it is only ever 8 weeks I suppose.