If you get an iFixit toolkit, the process of upgrading the RAM in the Mini is really easy. It was actually a lot easier than what the guides made it look like. I might have been lucky, but nothing on the Mini was gorilla tight from the factory and it disassembled with ease. I was mostly concerned about reattaching the wifi connector since it is so small - but even that popped back on first try.
I watched three youtube videos on how to do it - the iFixit one, the MacSales one and one from Apple Insider I believe. I decided I liked the MacSales video the best and watched it on a laptop as I worked on the Mini.
If you want to find someone to do it, I would look for a shop that is an authorized repair center for Apple, this way your warranty would still stay in tact. I'm not sure what shops you have by you, but I think even some online shops like B&H Photo here in the US will install RAM for a service fee.
Good Luck!