I've attempted to get enlargements from both digital and negatives through the mail via several places and the quality just ain't there. I have learned the hard way that its best to just take it to someone that actually looks at the end result and ensures it was done correctly.
These guys are equipped to do some complex stuff. Its not cheap by any means but at least it will get done correctly the first time.
I highly recommend Ivey Imaging.
They have yet to fail me.