It's a tough call. But I err on the side of fair but firm.
I have/had recently a customer who has deteriorated.
Over the last year it's gone from taking five minutes to get the cheque book, write it slowly and incorrectly to ten minutes of rummaging to 'I don't have a cheque book or cash until my daughter gets me a new one.'
Last time I cleaned I said not to worry pay me next time. Let me write it down for you and you can get a cheque written for two lots next time.
I wrote it down and put it in an envelope for Mrs. H and family. It had contact details and bank details if she/they wanted to deal with it.
Went back 8 weeks later and nothing had been done. So I didn't clean and said that not to worry but as soon as I get a cheque or payment for the last clean I'll come and do your windows.
Either she hasn't told her family or they don't value my service to bother.
Either way I'm not going back. It was becoming that it would take half as long to get paid as it was to do the job.