These are the two types of containers, of which I've bought hundreds, for ink sample giveaways.
On the left, nominally 1.8ml sample vials, which can in fact hold in excess of 2.1ml of ink, with removable and replaceable threaded caps in different colours. Under each cap is an O-ring, which is just about the right size to fit around the section threads on a Preppy,
On the right is 1.5ml centrifuge tube. The hinged lid can be re-closed several times, after which I expect the thin strip of plastic may tear or break along the crease. For sending the equivalent of one or two ink cartridges' volume of an ink to test, these would be perfect… but for the fact that, in my experience and from my observation, about a tenth of these are defective and don't seal liquids well enough to make it across the oceans without leakage.
Both of these will fit inside a cardboard mailer for the closed package to fit inside the maximum thickness of 20mm for a large letter. (That's how staff at my neighbourhood Australia Post office tells me to send the contents, even internationally; such a thin package does not count as, and would not be treated as, a parcel even if I paid Parcel Post postage.)