Dear Customer,
I have been cleaning your windows for <x time> and in view of the current economic climate have not increased prices for <y time>. In order to hold down prices, it is necessary for me to reduce my business costs where possible. Therefore, from <z date>, it will be the customers' responsibility to supply the stationery and postage required to send a cheque. Like all other businesses, I depend upon maintaining a reasonable cashflow therefore payments should be made within <a> days from the date on the bill. Should you wish to avoid postage charges, it is possible to pay online free of charge. If you wish to start doing this, please let me know and I will supply my bank account details.
Any that don't go with this by the end of the year say, try to replace them is my suggestion.
I lost a few when I became more businesslike about things. There is always some resistance when someone tries to make a change. Sometimes, it is necessary to leave a few behind when you make the changes.
As for the 50p a cheque charge, have you considered either a) starting an Abbey account (not recommended as they can be a pain) or (b) joining the FSB for £100 a year and starting one of their free business accounts with the Co-op.
Being an FSB member covers you for the accountancy charges incurred if you are ever investigated by the IR. It's worth the money just for that but there are other benefits too.