iFixIt is offering the Mac mini Late 2018 Memory Maxxer RAM Upgrade Kit. For $164.99, you get a PC4-21300 16GB RAM chip; a spudger, opening tool; angled tweezers, precision bit driver, and 4mm precisions bits.
Apple will upgrade your Mac mini to 16GB of RAM for an extra $200 at the time of purchase. iFixIt says their kit can give you 20GB for $35 less. If your Mac mini has two 4GB RAM modules now, you can keep one and add the 16GB module for a total of 20GB.