Before you decant
Updated
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One: what is the liquid, in regulatory terms?
An ordinary cosmetic, a flammable liquid, a product with an ethanol or other threshold attached, or something required to be sterile. Those are four different answers and only the first is a pure packaging job.
Two: which size limit binds?
The hazmat ceiling is 500 mL per container with a 2 liter aggregate per person. The TSA carry-on limit is smaller. For a product sold as a carry-on travel size, design to the TSA figure and check it at source rather than from memory.
Three: choose the closure for pressure, not for looks
A lined screw cap resists an internal pressure difference better than a flip top or a spout, because the sealing force is distributed and mechanical rather than relying on a small snap fit. That is the whole closure argument on this shelf.
Four: test it by flying it, or by simulating the change
Fill, close, and expose it to a reduced external pressure while laid on its side. A bottle that holds in a bathroom tells you nothing about a wash bag at altitude, which is the condition the product exists for.
Leave headspace, deliberately
Air in the bottle expands as the cabin pressure falls, and a brimful bottle has nowhere for that to go except out past the closure. Filling to a consistent level below the shoulder is a real control.
And label it, even for personal use
A decanted liquid in an unmarked bottle is a hazard communication problem at home and a screening problem at an airport. If you sell the bottles, selling a label sheet with them is a genuinely useful addition.
Keep the four answers together
Liquid classification, binding size limit, closure type and the pressure test result. A change of contents reopens the first and often the third, which is the shape every property here arrives at.