He's not just being polite but he made the conscious decision that this store is the one he wants to support because they were willing to lose revenue in order to help the customer.
The phrasing of the third panel confused me too, until I realized he already got the peaches he wanted (if not the right brand), they're on the counter, and he's now ordering MORE
I'm really enjoying these. They show that he isn't just about beating anybody up but he actually has a code that he lives by. If he beats up everyone, he's just 1980s Heathcliff. By offering these counter examples, we see a much more complex person.
This is true, and there was a joking element to my comment, though I do think if you happen to start with these two, expectations will be mis-set in a very jarring way. For those folks, poor Everett is about to snap.
Does he value the honesty of the shopkeeper that much, that he then, instead of going to the competitors store, buys whole lot of the "wrong type" of peaches from the honest seller?
No, he isn’t always wrong, that’s why I mentioned the filtering part. More often than not the comic presents a reactionary, violent jerk or someone who says shitty things justified by “that’s how we all really feel but won’t say it out loud” kind of thing.