It's been a while since I've been on here - rubbish weather has temporarily stopped me from working...
Thought I'd share this letter I sent to a  serial late-paying customer after he had the cheek to dump me with a note saying they were fed up with being asked to pay their bills and why don't I come back in the evening to collect:
Dear xxxxxx,
Window Cleaning Service
Thank you for your payments that have now brought your account up to date.
I acknowledge your wish to terminate the window cleaning service, although I must emphasize that the feeling is mutual.
You state in your covering letter that the number of prompts and tone of letters requesting payment has become tiresome. At no time have I been anything other than professional and courteous in my communications with you or your wife. 
Communicating with your household via landline (I do not have mobile numbers for you or your wife) has proved quite difficult as often the phone isn’t answered but instead goes to a message that says that a message cannot be left! 
When I managed to speak to Mrs xxxxxxx in August so as to remind her that that the bill from June was outstanding, she promised a cheque would be sent prior to you going on holiday. I was not surprised when it did not arrive. Nonetheless, I patiently waited until 3rd September until my next visit. Remarkably, whilst I was cleaning the windows to the rear of the house, xxxxxxx  got into her car and drove away without acknowledging me or even apologising for the delayed payment. This was not the first time this has happened. In the circumstances I believe I was even more patient and courteous in carefully writing out the compliment slip requesting the two cheques. Another 10 days passed without receipt of payment. So another reminder letter had to be sent. I do not believe that is excessive. 
On several occasions in the past when Mrs xxxxxxx was in when I have come to clean the windows she has not been able to pay me as the cheque book was with yourself but was given assurances that it would sent in the “next few days”. This never happened without having to chase for payment.
For the first couple of years I always left a stamped addressed envelope for your convenience with the bill. Because it proved to be a waste of money for the stamps I decided to stop stamping the envelopes but still left an addressed envelope.
For your information I have tabulated below the cleaning and payment-received record for xxxxxxxx
Date of Clean             Date of Payment   Days to Pay
03 Sep 15                    25 Sep 15                       22
26 Jun 15                         25 Sep 15                       91
01 May 15                   01 May 15               0
06 Mar 15                    01 May 15               57
07 Jan 15                            04 Feb 15                       28
06 Nov 14                     04 Feb 15                     90
11 Sep 14                          01 Oct 14                      20
23 Jun 14                          05 Sep 14                      74
28 Apr 14                          05 Sep 14                      130
04 Mar 14                          09 Apr 14                      36
27 Dec 13                     09 Apr 14                     103
29 Oct 13                        21 Nov 13                     23
02 Sep 13                         21 Nov 13                     81
26 Jun 13                         03 Aug 13                    38
01 May 13                 01 May 13                     0
05 Mar 13                         02 Apr 13                     28
08 Jan 13                         02 Apr 13                     84
13 Nov 12                         08 Jan 13                    101
12 Sep 12                         03 Oct 12                    21
19 Jun 12                        19 Jul 12                    30
24 Apr 12                        24 Apr 12                    0
24 Feb 12                        24 Apr 12                    59
25 Jan 12                        04 Feb 12                   10
The average time it has taken for a window cleaning bill to get paid at xxxxxxxx is 49 days or 7 weeks. My payment terms are clearly stated on every bill as 7 days – certainly not 7 weeks! The odd late payment is always to be expected; holidays, sickness, mislaying the bill etc. But over the 3½ years and 23 visits to xxxxxx I have been paid on time on only three occasions.
I have over 300 regular window cleaning customers varying between 4 and 8 weekly clean schedules. During each year I also have dozens of “one-off” cleans for conservatories, gutters, patios and driveways.
With such a busy schedule I neither have the time nor inclination to re-visit customers in the evening to collect payment. Instead, I make it as easy as possible for customers to pay their bills either by cheque (self-addressed envelope for their convenience), by bank transfer or by debit/credit card via PayPal. Therefore, “tiresome” as you say, is exactly the word that I would use to describe the on-going situation of having to wait weeks and weeks and to repeatedly chase for payment. Other words would be “frustrating”, “embarrassing” or “soul-destroying”.
Those customers who respect the professional service I provide all manage to pay their bills in a timely fashion. Should you decide to employ the services of another window cleaner in the future I hope you will show more respect for his service than you have done for mine.
Therefore, and I make no apologies for using your phraseology, please take this letter as notice that I do not want to continue cleaning your windows and, yes, we are (indeed at last) square! 
Yours sincerely,
I know it took me an hour to compose the letter - but I felt so good after!!