We do similar depending upon amount outstanding, if it's less than £10 we just leave a note saying the windows have been cleaned today, then either go back later to collect or get it next time. However if its more than £10 we leave a note saying:
"LIONHEART CLEANING"
Your windows have been cleaned today.
Amount Outstanding: £______:______
Please make cheques payable to “LIONHEART CLEANING”
Post to: XXXXXXXXX
Tel: XXXXX XXXXXXX
The reason for the £10 level is due to banking charges for cheques 25p per cheque, which on a £5.00 cheque is equal to 5% of your earnings, we do the same if people are in and want to pay by cheque, minimum £10.00 and we'll deduct it from your next clean.
We never leave envelopes or stamps and never get anyone who gets more than 2 payments behind.
Works okay for us.