No sympathy here... I work in retail and meet people like you all the time. All you have to do is make one little mistake or an oversight and people just can't wait to scream "poor customer service" and use that for leverage to get something for nothing. So they made a mistake and you saw an opportunity, got on the phone to see what you could get for nothing...cause I mean really how were you inconvenianced' you had one mag that you could use while waiting for the other 3 to arrive, they were just late, its not a life or death situation here, just a couple of mags. Really there is no inconvenience at all. So the guy offers you 10% and a tee shirt, which is pretty darn reasonable, you accept the offer and then you decide "nope I want more" so you called back and said give me a free mag,(25% discount, which is a pretty big discount) chased the issue through a number of people, annoying them to no end and taking them away from servicing good customers and because you didn't get enough free stuff you were crying poor customer service.
I think the 10% was reasonable,they offered, you accepted and then you tried to get 25%, they wouldn't give it to you and they are the problem? You are the problem my friend, not the retailer. You got your mags, you got them for less than you expected to pay and you got a free toque to boot. sounds like you made out pretty well.
But you're upset because they didn't give in to your unreasonable request for a 25% discount cause they made a mailing error. I can guarantee they lost money on this deal with all the man hours involved, 2 mailings and the discount, I'm confident they are not upset at all that you won't be back.