I primarily send people feedback about their mobile applications, but I've dealt with my fair share of website feedback as well.

Most notably is the San Francisco federal credit union website. They updated their mobile app and website last year and the developer who built it had absolutely no idea about accessibility at all, and subsequently locked me out of banking because of their ignorance. The credit union themselves brought me in to show them exactly what was wrong when accessing the website and the app, and we ended up having to go to mediation with disability rights advocate lawyers to get them to start remediating everything. It will be fixed in 2.5 years :-(

Due to my disability, I ended up going on Medicare last year. Getting dental insurance through Medicare was an absolute pain in the ass, as the broker I had to go through had a terrible website. Calling them and complaining resulted in nothing happening whatsoever, and they showed no sympathy when I explain the situation. Definitely a "well that's just the way it is" attitude from everyone.

Smaller websites like Zeel and BloomThat had websites that worked relatively OK before making really stupid visual updates that once again broke everything in terms of accessibility. After quite a lot of back-and-forth, nothing has come of any fixes and it's been over a year now even after all the follow-ups. They seem sympathetic, but definitely they do not have the understanding of how to properly billed an accessible website. Just have to keep making enough noise to get noticed and show people that all of us are trying to enjoy their services and not just people without disabilities. I have found that mobile applications seem to update and move faster with accessibility feedback rather than websites; hell, complaining about Lyft on Twitter got me my current job. :-)

Edit: wanted to add my experience with cards against humanity. I used to have no trouble ordering new updates from them through their website, but something happened recently where the shipment option was unable to be activated using voiceover either on mobile or with a computer. I was trying to order the geek pack, and when I was unable to complete my request, I sent them constructive feedback about the issue and asked if they could just send me one and use my currently existing payment information to buy it. They were extraordinarily apologetic and went absolutely above and beyond and ended up sending me every single expansion, the bigger blacker box, and the base set along with the pack I originally wanted. It was a tremendous care package that was unexpected. I put braille on all of those cards since I already owned them all and donated the set to the lighthouse here in San Francisco. I haven't had the need to order anything recently, so I am unsure if they have fixed this problem yet.

There are Web Content Accessibility Guidelines.

Send a message like

Dear Admins,

I found the following issues on your website. (INSERT ISSUES HERE). Would you resolve these issues. For future reference to reduce the number complaints and make your website open to more users to increase the popularity. Please visit
https://www.w3.org/WAI/intro/wcag. This would make it easier and efficent for users and increase the experience.

Thank You for your understanding.

I have a couple times. Usually when I try to report accessibility issues, the contact link is either nonexistent or extremely difficult to find. However, I was able to email the famous Khan Academy to inform them that I was unable to navigate the website, and all I got was some automated email saying that they are doing everything they can to make the website better.

I've sent feedback about accessibility and how to improve it on some sites.
Also good feedback on surveys from sites that work well.

Usually for feedback of some site nit working I don't expect feedback.
The usual format i use is just telling them why I didn't buy from their site, explaining the problem.
Eg. The site does not work with voiceover, it's impossible to navigate, or there is no description of items or colors. For a clothing site that might be having 5 dots showing a dark color but no description of the color so no clue what kind of color it is. Even something as simple as an item being black or white: describe it! Tell me boutnthe design and size for that particular item.
Or missing buttons (how i do search? What is that button, button, button?). So many little annoyances.

I wrote to Amazon about their method of viewing comic books on their Kindle Fire "7. I asked if there was any way they would be making if more accessible ( such as zooming in on panels more then the user was able to ). They wrote me back and told me they didn't have plans to add accessibility to that function .

I now use the kindle app, and apple'a accessibility features to read comics on my IPad Air .