In the last weeks I have ordered some PCB's at jlcpcb.com and I must say that I am very happy with the results, it took about a week from upload to delivery here in switzerland for a very reasonable price ($30 including DHL shipping for three designs at 5/10 PCB's each)
Exporting with 'Prepare for Manufacture' works just fine (I first manually renamed layers buth this seems not strictly necessary).
Two of the pcb's were symetric, so I had zero point in center and was unable to properly upload them.
The problem is that drill coordinates cannot be negative, as soon as this happens the upload page of jlcpcb does not work correctly anymore, board size is gone and you cannot see the gerber preview.
The problem went away after I put zero left and below the first drill hole to keep jlcpcb happy....