Are you the poster that had leaking roof after gutter guard installation?

Would you have had $374 upfront if you got the bill at the time the work was done?

Plumbers charge $100 call out fee here, $374 doesn't sound bad for the work he had to do.

  We have been asked to pay $100/month for next 3 months, and if not, the account will be sent straight to a debt collector, after which we will be required to pay extra fees for the debt collector + interest. 

That sounds reasonable $25 p.w. Customers can't dictate the terms they will pay accounts/bills even if the bill was late in arriving. You have benefited from having the work done. The plumber has met you 'half' way and is not demanding total upfront .