Yep, custom order or your shit outta luck. The 8GB is... fine, but not really an upgrade unless I was doing something that didn't tax ram at all. I tried it out when mine was in for repair, and I did notice the increased performance of the M series chip, but it showed it's limit pretty quickly when I spun up the 8 docker containers or w/e I was using regularly.
There was sale recently in my country across online retailers like Amazon. There was decent discount on 8GB one, but the 16GB one? It’s not even available. It is never available with retailers.