Has anyone used an online printshop or a service to create a metallic photo collage? I'm looking to create a collage for my dad as a Christmas present.

Any recommendations would be much appreciated!

I would probably look for someone who specializes in photo editing (photoshop) and have them generate a print-ready file vs looking for a printshop that can throw something together for you. Those are two different worlds in my opinion. You might want to look at fiver.com and type in photo editing and you'll see a ton of people to choose from. You can see rates, read reviews, see prior work, and decide from there.