i would just go straight to it.. from the start i just printed envelopes with my address and business name in the corner, then printed small notes both sides - front with name...your windows have been cleaned today... something like that, with a space for the date, and amount due. i have a little picture and contact info with my website etc.. and on the back how they can pay, payment is due within x amount of days, non payment will trigure small fee. Who to make cheques payable too and says to write from address on the back of the cheque, your regular cleaning services will continue between... etc
i print 9 on 1 a4 page, and slot it in the back of the envelopes - with a first class stamp stuck on or not your choice, and post it through the door after cleaning, if theyre home i just knock and they usually pay cash after or i leave the envelope with them all ready to send a cheque..
i havent had any missed payments, and usually receive the pretty little envelopes through the door within a week with the nice cheques that take ages to clear!