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In USA, adding RAM will not "void your warranty". The Magnuson-Moss Warranty Act assures that. However, if you damage something that damage isn't covered, and Apple can require you to put the original memory back in if it does need service - so doubling your chances of accidentally breaking something.

So - consider the cost savings vs. the risk, and make your decision.
 
Apple have updated their official support document for Mac mini RAM.

The RAM in the 2018 Mac Mini is not classed as user replaceable. It is required that Apple or an authorised dealer do it :
https://support.apple.com/en-gb/HT205041

Add to this I and various others have been told point blank by Apple support staff that opening your Mac Mini 2018 yourself voids the warranty.

Just saw this as well. As I got AppleCare+ for my mini if I need to upgrade the RAM before the warranty is up I’ll just take it in to Apple. No big deal. After the warranty expires, though, it’ll be nice to be able to upgrade again myself if I need to.
 
From this page:
https://support.apple.com/en-gb/HT205041

Screen Shot 2018-11-12 at 10.02.10 AM.jpg
 
Adding your own RAM in the Mac mini does not void the warranty, however, if you are worried about it then simply save your original RAM and reinstall it before taking your mini back to the Apple store.
 
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