Elsa,
Perhaps your shop has a print profile for the aluminum medium ?
I have printed only a few times, because the experience was disappointing, a printer came to give a talk to the photo club and offered the service to:
1. calibrate your working screen (although I have a spyder and I do my own calibration every 4 weeks)
2. install the proper printer profiles so you can proof the images before sending them to the printer
During the talk it was also explained the advantages and limitations of different mediums
I have not printed on any fancy medium like canvas or aluminum , but what I have sent to them , looks exactly as I wanted it