Erich,
I see from your profile that you're a fellow Geordie and new to window cleaning.
You will find - when you first start up - a large amount of 'bad customers'.
And no doubt money is tight; and for every £10 quid you're owed and don't get when you've got bills and food to buy, it can be frustrating.
You've obviously got a computer, so make sure you put on your 'Your Window Cleaner Has Been' chits when you're going to collect.
I still collect from my estate areas; so I put something like 'I'll be round to collect on Saturday morning between 9.30 and 11.00 am; if you won't be in or do not wish to be disturbed, please leave the payment under an object on your doorstep'.
If they're not in and no payment is left, I leave a SAE with payment instructions.
If I don't get paid; I still keep on cleaning; upto three cleans, then leave a firmer note saying basically that I've cleaned your house for three months, you haven't coughed up, I won't clean this month so your bill doesn't mount up; please cough up.
They usually do.
If I feel they're extracting the urine; drop 'em straight away.
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Also, I always have my customers names and phone numbers. Once when I called to collect I knew they were in, they just weren't answering the door.
So I walked round the corner and phoned their landline from my mobile.
The lady of the house answered the phone and I told her who I was and that I'd just been knocking for payment. She paid (by cheque); looking all embarrassed; saying she thought I was a salesman (yeh right).
Dropped.