There were two men that did the job. If there was only one it could have taken twice as long.

The plumber said they could pay it off at $100 a month x 3.... there would be no sending to collection in 7 days if that offer was accepted.

and the explanation of the OP is not very clear about what work was actually done but I read it as there was another issue on the OP's property that needed attention and the OP asked the plumber to do the job there and then..........and come back to do another job, which he did not return to do.

Did the job really require two men or did he just have his son along for the work experience?

Who knows about the son, you wouldn't think 2 plumbers would turn up for a small job, but  they may not have known how big/small the job was till they looked at it.

The OP has not disputed the amount of the invoice with the plumber, just the lateness of it being sent out and wanting to pay it off over 3 mths.

Next time he can bring his wife along and charge 1/2hr x 3

????

No one here knows exactly how much time the job took, plus he used a digger. His invoice could be quite reasonable for the job he did. We also don't know exactly what the plumber did at the neighbours house. Bit strange if 2 separate jobs in 2 different properties would require exactly the same amount of time to fix.

A plumber here would charge $150+ to change a  tap washer.

The thing is, it appears the price charged was not the issue. According to the op, it is the late bill....so she wants to string out the payments to teach him a lesson.

Besides, the plumbers wife is busy working in the office.

The plumbers wife isn't very good at getting the invoices sent out is she? 

The plumbers wife may not have been told about the job until then............. speaking from experience (but my OH is not a plumber)

I find the charges excessive (1.2 hr x 2)  and his way of operating pretty slack but I would also hate having that bill around for 6 months just to prove a point, I'd pay it and be done with it and then let friends and aquaintances know how he operates, word of mouth can be a wonderful thing.

Just on a couple other points.  I get sick and tired of people asking my hubby to do cash jobs. No.  I also get sick of hearing "we are pensioners" or "we haven't got much money". Well sorry, we have to put food on the table and have bills to pay, we are not a charity, want something done, save the money and then get it done. That may sound harsh but when you hear this nearly every week you get sick of it.