I feel your pain, as I have been stiffed for payment more times than I care to remember. I would carefully arrange a time and date for collecting, only to find the customer says she still has no money/has forgotten/gone out. I would call a few more times to find more of the same. This would do my head in. I have since developed a coping mechanism which simply now tells me to now just let it go. It's only cost me some time, but I am not prepared to waste any more of it on vermin who want something for nothing.
I have one ex-customer who still owes me for a clean I did for her over 3 years ago. She has recently asked me numerous times for me to call to clean hers again. I always politely remind her of her debt, and that I would be happy to clean once the money owed plus advance payment is in my hands. She always promises it will be, but I am still waiting. I now find it quite amusing, as she is the one now chasing me for a clean which will probably never happen!
Try to turn the negatives into positives. Your 'customer' may have won the battle this time, but you will win the war.
John