Equally strange that my original post is not there. :twisted: :roll:
In any case.....with the lab that I use, I create different files for different size prints. For example: I want a 16x20, an 8x10 and a 5x7 of the same image.
I take my 'working original' (the one I've made all my PS adjustments to) and crop it for a 16x20 at 300dpi (as per my lab's instructions) flatten and save as newfilename.jpg (or whatever you want to call it).
Then I go back to my working original, and crop for the 8x10 at 300 dpi. Flatten and save as newfilename2.jpg.
Do the same for the 5x7, and then upload them to the lab.
Your lab may have a different way of doing things, and it may be worth your while to hound them until you get some answers.
If it were me, though, and they weren't 'very helpful'.......I'd be finding a different lab.

YMMV.