Packing and Shipping Art

Alright. I'm fed up. It's been a year, and about a dozen paintings later, that I feel like I'm getting overcharged every time I send off a large painting. I have been going through Postal Annex the last two times I have mailed an 18"x24" painting. The first time, I stepped in with the painting, and said I needed a box to ship that would fit this painting. The woman stepped in the back with the painting, and came back with an ultra packaged, bubble wrapped, package peanuted, super protected box. I wasnt expecting all that, I just wanted a box that would fit it, and I'd do the dirty labor. They charged me nearly $20 for all that madness, and I swear that on the itemized receipt, it stated "SMALL ART BOX $2.99." So today, now that I'm shipping off another painting, I prepared myself. I didn't bring the painting, I just stepped in and rattled off the dimensions. She said the art box would be $12.00 this time... Whaaaat? I even said, "I would never have imagined a hunk of cardboard could cost so much." This is definitely the final time I will ever step into a Postal Annex.
You know, it would be one thing if I were charging five times the price I do for my paintings, but I'm not. I'm trying to be as fair as I can to my clients, without seriously cheap-skating myself. Currently I charge $25 an hour, plus price of canvas and a new tube of paint or two for paintings. Then I add a $25 shipping fee, which covers about $20 from the Post Office including shipping confirmation. I think it would look very suspicious if I bumped the shipping fee any higher than $25, just because places like Postal Annex are charging me an arm and a leg for corrugated cardboard.

So now, I am asking all you great artists and creative thinkers, what do you do when you ship a piece of art of this size? Where do you get the box? I've once made my own box by cutting down a giant tv box to fit. But it looks a little Frankenstein-ish. I don't want to give the visual impression that "I'm going to send you a work of art in a very scary box that could fall apart at any given moment in route." Where else does this leave me? Should I check Office Depot, or Staples for a more fair price point on boxes? Or should I just charge $37 for shipping? That makes my stomach sink, just typing that large of a number.