No, I have never filled out a complaint form. In my experience I find that if I can't solve a problem verbally, then there is little point to pursue the matter as it will involve too much of my energy. Thankfully I haven't been in that situation too often. Once I purchased a prepaid visa card for $200 from the post office and only realised the next day that it was out of date. When I took it back, I had to prove to the post office that I bought it there and all kinds of problems started to unfold. It took almost a month to solve this problem. I am always careful now.
I imagine you no longer had the receipt? Although even in that case you could get your bank statement which would have a $200 payment at that post office the day before.
I filed a complaint for a laptop I purchased one time. The reason is because it kept shutting off after ten minutes of running! When I brought it into customer service they told me there was nothing they could do about it. Didn't think twice about filling out an official complaint after that haha.
I once filed a formal complaint with Walmart because they overfilled my oil during an oil change by a quart. I had to have a quart drained. This could have caused severe damage to my vehicle.
I complain about substandard food. I expect things to come as they are described on the menu, be properly cooked and amended to my personal requests. My partner hates this. He will complain in stores and I won't, but I complain about food while I'm out and ask for changes to what is specified on the menu and he doesn't do this.
Just recently I complained on a public website about Nestle's Dulce Gusto coffee machine. Not exactly about the machine itself, but when I bought the machine, it came with a voucher for 2 free coffee boxes. No expiration date was mentioned anywhere in the voucher. Then, much to my surprise when I tried to get the free boxes on the website the offer was expired! Long story short, I called them, they didn't accept to send me the free boxes because the offer has expired. After I complained about it on a public website they called me in less than 24 hours offering to send me the 2 boxes as a courtesy!
Sometimes that's the only way to make your complaints heard, companies are terrified of seeing a public criticism available for everyone to see. Did they ask you to delete the post or something?