#SEYBOLD
The OVERPRINT command in Illustrator will ONLY work on a separation…when
you Preview or print the image (on a color printer), any "overprinted"
objects will be opaque (like normal).
The fact is that PostScript (the language, that is) doesn't support
transparent color; when you fill/stroke something, it completely covers
whatever it's on top of.
Transparency _could_ be acheived through creative programming, but it would
get VERY complicated (if you've ever creaed a transparent or reflective
effect yourself in Illustrator, you'll know what I mean). It would
essentially entail having illustrator mask off the "tinted" area and adding
a percentage of color to every object in the clipping path…this could
mean MASSIVE amounts of printing time, however.
So don't be hard on the vendors…it's not their fault.