I use TFSource, but their Order Stacking system seems to be the same. I never pay extra for the mint box, because honestly, even the packaging that I do keep tends to go into a box or cupboard somewhere. If you're buying to keep these sealed, it might be better to pay the extra, but I can't think of having received any packaging with any noticeable problems, and the toys inside are my priority.
I actually thought the "huge mistake" you were going to talk about is the propensity to go "oh yeah, just add that now, I'll pay for it later" and then having tons of stuff that ends up being really expensive. I did have a huge cost of items in my stack and had to resist adding any more until I shipped and paid for my last lot, for fear of ending up with too much that I couldn't afford to pay for.
assembling a Neo-G1/TF:TM cast. Please PM if you have (or know of) the following at a reasonable price: Classics or Henkei Prowl, Astrotrain, Ramjet, Skywarp, Sunstreaker, Sideswipe, Blurr, Perceptor, GDO Wheelie and Springer, or 3rd Party Motobot Arcee, FP Springer, iGear Ratchet and Ironhide, City Commander (and/or Ultra Magnus), Starscream (deluxe) Coronation add-on, or Spike and Daniel in their Exo-Suits.