I'd like to put in my experience here....
DH & I own a landscaping company & have numerous residential accounts. Some of these customers prefer not to be invoiced monthly, but to pay at the time of service or whenever they happen to be at home when our guys are there servincing their lawn, etc. and they receive a hand written (carbon copy) invoice on the spot. In the past, we had a customer who paid on-site, then called a few days later saying that they no longer needed our services due to a relative who was going to take over their lawn care...fine, no problem. Well, their last check came back "ACCOUNT CLOSED"....when my husband went to their house, no one would ever answer the door, nor our phone calls. We went the certified letter route....but they refused to accept it, saying the person the letter was addressed to no longer lived there....LIE. So sending a letter, ceritified or not, is not likely going to help in the resolution of your problem. If someone wrote a check on a closed account....9 times out of 10 they were very much aware of what they were doing and are unlikely to answer your calls or accept your letters. You will probably have to involve filing a complaint with your local police department in order to get the ball rolling and get some results. Also - you're probably not the first person she's done this too. That's what we had to do....when the police knocked on the door, they didn't take the "oh..they don't live here anymore" crap and stepped in the door to see the person sitting in the kitchen hoping they wouldn't be seen. All we wanted was for them to make good on the check and pay us what bank fees we incurred....with the police in front of them & jail time looming, they handed over the cash on the spot and 2 hrs. later we were called to the police station to get our $$ and fill out the paper work. We were only talking about $175 here...not a ton of cash, but worth it to us.
I don't mean to seem like a "gloomy gus" here, but I'm only realistic. It's very, very rare that someone writes a check on a closed account and then just doesn't return your calls "by accident" or some other reason. If you haven't heard from her in 2-3 weeks, I'd say go file a complaint with the police. Unless, it's a small enough amount that you're willing to let it go & write it off. And as far as her bad-mouthing you if it was a mistake...that would just be wrong. If she hasn't answered your repeated calls, it seems she's avoiding you....all you're doing is being an astute business woman and handling your business...and for people who don't write bad checks, what would they care?? You're not in this for free, right??
