Kevin, you have heard the statement, you can’t fix stupid haven’t you? Hate to say it, but you are always going to have those people who don’t read the info. provided, don’t look at the photos provided, etc. whether it is provided in the online description and/or the printed information sent to them. Some don’t take the time, others misread or miss it (we are all guilty of that at some time or another no matter how closely we read and look at things.) As you note, if you have only had two of those in all these years, that is pretty good odds compared to all your sales. I am not sure what else you can do if you already show photos of the back, provide detailed descriptions and all. Video may be a good idea, many people are more “visual.” Even though you cannot link to it from Amazon, you could provide the link to the video site with the directions printed and mailed with it.
Sometimes, we try so hard to cover all the “ bases” that we put too much info. out there and people then only half read something or ignore it all together so perhaps, less info. can be more. The other thing would be to put the most Important information first so they don’t miss it if they skip half of it.
Unfortunately even with that, like you noted no matter what you do, you are still going to have those few!