If you want to keep on doing the job leave it til next time around and see what happens.
I had a similar situation in the past, I agreed a price for a regular monthly clean over the phone with a woman as I worked in her road and knew the property .
I cleaned the windows and left an SAE for payment. Nearly 2 weeks went by and had not received payment. Then one evening I got a phone call from the womans husband asking me why I was charging £2 more than their old window cleaner.
I wasn't very happy to say the least and I explained to him I had quoted the price to his mrs upfront before doing the job and she agreed, and that he was out of order.

I waited for the cheque which arrived a few days later and was going to take them straight off the round considering them to be messers.
Anyway the next month came round and I decided while I was in her road to clean them and see what happened.................
1 year down the line and they have turned out to be ok, I never see them when I clean (they are out working) but payment is always left out for me and I have had no problems since that day.