ejwebb made a great point - no color correction! If they look good on your monitor then add "No Color Corrections!" on the CD and envelope to make sure they see it.
Now I'm pretty sure that you're monitor isn't calibrated, but this is still what I recommend. If your test shots come back a little under-saturated then you know to add some saturation on the next run. DO NOT trust them to saturate the same amount every time!
Which brings up another point. Try giving them 4 or 5 of the same image done different ways. I will do things like add 7% saturation on 1 and 12% on another. I'll also do different sharpening techniques on the same image, like 120%, 140%, 160% and so on. If you do this, put that info INTO the picture - just add a text layer with the settings you used. When you get these samples back you'll know just what to do to your images to get them back the way you want.
When you get the pictures back, check the back. Many (all?) photolab printers will print out a code on the back stating the color corrections and exposure adjustments. You might see something like "N003111" which *could* mean anything, but it *does* mean that they made adjustments. If you told them "no adjustments" then that string should be all Zeroes.