@TomTomBinks,
Personally, if I felt it was necessary for me to make my thoughts known to my neighbors concerning their flying Confederate flags, I would visit them.
Assuming you are able to a) Approach them in a non-confrontational manner b) Control your emotions if you hear something you don't like and c) End the conversation as soon as you perceive it is turning into something ugly and simply walk away, visiting your neighbors will offer the opportunity to hear what they have to say and possibly engage in an exchange of ideas. It might also offer the opportunity to get punched in the face and start a neighborhood feud so there's that to think of.
Have you decided that you know their reasons for flying the flags and can learn nothing from talking to them?
When I lived in Charlotte NC, our next-door neighbors were a couple who were originally from Alabama. Several times a year they would hang an enormous Confederate flag from the balcony of the the second floor of their custom built plantation style home.
As I've mentioned, I don't often talk to my neighbors and these folks seemed to share my philosophy even to the point that they wouldn't acknowledge a friendly wave from me. At one point though, we were forced together when we both attended a Zoning Board meeting concerning construction in the publicly owned greenbelt behind our homes. According to the unique terms of my creed, we were outside of the neighborhood and so I was able to interact with my neighbor as I would anyone else, and I struck up a conversation in which, to my surprise, he participated with passion. We found we agreed that if the land was being developed by a private owner, we would still not welcome the construction but we would not be attending the meeting, however we were both angry over the fact that the city had pledged to always maintain the space as a greenbelt but were now reneging on that pledge. Most of us would not have purchased our lots if it weren't for the pledge.
One thing led to another and I learned that we shared a number of conservative views and I imagined that I might end up bending the rules for this guy and his wife. At some point he told me that he participated in Civil War re-enactments throughout the South. Apparently he was the ancestor of a high ranking Confederate officer (the name was unknown to me and forgotten after 20 minutes, let alone 20 years.) Clay, the neighbor, always played this ancestor (when historically permitted) or an officer from an Alabama regiment. According to him, his ancestor had fought in the battle of Gettysburg (the annual Superbowl of reenactments) and that was the event he most looked forward to as it drew thousands of participants.
It was truly fascinating, as Civil War reenacting is a rich and complex subculture which you might want to look into just to provide a perspective on why some people fly the Stars & Bars. Clay was a hardcore reenactor who totally immersed himself in the experience, (eating hardtack and salted pork during breaks for meals for example) and had disdain for the useful (they filled the ranks of "extras") idiots (they didn't care about the history) who treated the events as an opportunity to dress up in cheap, historically inaccurate costumes and play "Army."
As I had expected, Clay flew the flag to commemorate key events of the war, and what I didn't realize was that he was seldom home during these times. Because the Zoning Board meeting was late to get started we had a fair amount of time to talk, I had the opportunity to form the opinion that while Clay was of the mind that the Civil War was about more than slavery he wasn't overtly racist. He was, however, an obvious bigot concerning Northerners. He seemed to give a pass to those who were fellow hardcore reenactors, but had low regard for all others. Back then, Charlotte was being overrun by transplants from the North to the point where it was having an effect on the culture of the city. I didn't like it (I hoped to leave NY behind and didn't care for it to follow me to Charlotte), but Clay
hated it. So much so that in the
speech he eventually gave to the board, he gratuitously lambasted Northern transplants and included those who may have embraced the Southern culture they found but still wanted their bagels and NY pizza (In other words...ME!)
I hadn't told Clay where I was from and while I didn't speak with a Southern accent, 20 years of living there had rounded of the sharp edges of my NY accent and I'd adopted certain uniquely Southern phrases and even a slight twang with certain words. I doubt I sounded like a local, but I could have come from just about anywhere West of the Mississippi river. It's possible that Clay wasn't including me in his tirade, but it's also possible that he was. In any case by the time the meeting and our conversation was over, he seemed to me to be a pretty obnoxious guy. While he gave no indication that he was a racist (and he wasn't shy about letting anyone know who he liked and who he didn't like), he, of course, could have been. However, by all indications, if he also flew the flag with the intent of getting up in anyone's face, it would have been Northern transplants. Obnoxious and bigoted, but not racist, but still enough to nip our budding neighborly friendship.
My point in recounting this story (other than I view a good life as a series of good stories) is Clay, on a very basic level, was a real life example of your neighbors: He flew the Confederate flag in a neighborhood which he had to know was not fully inhabited with people who shared his passion for the Confederacy and likely did contain some residents who would be offended.
I didn't inform Clay of my belief that regardless of his intent, the fact that the flag would almost assuredly offend some of his neighbors was reason enough not to fly one of a normal size, let alone one that could blot out the sun. I didn't because the opportunity didn't arise and I felt no moral obligation to make it. In retrospect, I'm quite sure that if I had, it wouldn't have even caused a tiny ripple in the pool that was his mindset, but that wasn't why I didn't make the effort. I didn't because while the flag had bugged me, I can't say it offended me, and I didn't feel that any of my neighbors who might have taken offense from the flag were incapable of conveying this to Clay themselves. I'd never been deputized to speak on behalf of anyone else in the neighborhood and otherwise never would have. I am also a committed proponent of private property rights and free speech, and both applied to Clay's flying of the flag.
By talking to Clay I didn't find out he was a great guy with some hitherto never expressed but benign reason for flying the Confederate flag, but I did learn that I wasn't living next to a Klansman, and I learned an awful lot of fascinating things about the Civil War reenactment community.
It's unlikely that if you visit your neighbors and lead with something like "I want to talk to you about your offensive flag," that you will have the opportunity to learn anything other than how they react to being offended, but given the proper approach you could, and you could still express you opinion. Writing a letter, IMO, will not provide any opportunity beyond satisfying your desire to make a statement and feel like you've done something you are supposed to do.
Obviously it's your call, but you've asked for advice. I don't think writing a letter is something to be warned against as blickers has, just not the best way to go.
Out of curiosity, if you do send the letters, will you save a copy to show to your like-minded neighbors?