I've read/heard C E Ferulic is one of the gold standards of the Vitamin C world in how effective it is but it oxidizes easily. Oxidized Vitamin C is also a form of self tanner... Plus it is expensive. They do sell a travel sized amber glass eyedropper bottle.

I've traveled with skin and essential oils in my 311. They do better when pressurized in a fully sealed container like a smaller plastic saline dropper + cap with nipple to seal it off or sealed essential oil (twist on cap with liner like polyseal https://www.sks-bottle.com/Cap-Liner.html). Rohto's Melano CC Spot Treatment (not marketed as an all over serum) from Japan has a pretty good sealing mechanism and sun blocking tube, plus the price point is lower but lacks the Vitamin E. Still might be enough while you're traveling, though.

This reddit thread has some other close enough dupes which includes Paula's Choice https://www.reddit.com/r/SkincareAdd...upes_only_two/