Photo Enhancement at Online Printing Sites

Recently I stated using LR to work on the images I take before printing them. I used to print them at an online printing service, Snapfish, without doing any enhancement/editing/adjustment, before uploading the files, and the prints always looked great.

The last set of prints I got from them looks very dark and the colors are off, totally different from what I sent them. This time I used LR to sort of "enhance" them myself. They looked great on monitor but the prints are awful.

I see there is an option in LR to optimize the images for print, there is even an option for the type of paper that is used for printing. Any advice on how to deal with this? Would optimization in LR using that option fix the problem?

I am not using a calibrated monitor but I am using the same monitor as before. Also, there is no option on Snapfish to tell them to print the files "as is".

Thanks,

Sedali