Definitely a legitimate complaint.
IF ANYONE went into this transaction with AN ARTIST who has never done anything like this before, thinking that the ARTIST could mail out 3800 comics in a timely manner, well packed, shipped properly, with timely communication, all high grade, without SOME potential issues, I would like to sell you some Batman New 52 comics signed by Bob Kane and Bill Finger. LMK, I'll cut you a good deal with timely shipping, good communication, great condition of comics.
I don't think any complaints about shipping speed are warranted yet, but I do think charging $8 and then shipping media mail (when Priority Flat Rate is what was advertised) is in poor taste.