Plastic retains the smell of whatever's been in it more strongly than, say, glass, and if it was 'nasty' liquid, reuse might leave the refill liquids a bit skunky too, or at least leave a fair amount of smell and taste from the previous one if it was strong. Just my experience -- I tried reusing an emptied, rinsed Dekang bottle a couple months back with TV liquid for my own use and figured that out.
3ml and up bottles for DIY and samples can be purchased inexpensively at
USPlastics, they also have glass dropper bottles. (
Candlechem on ebay is great too, but they're on vacation til March 8th.)
I often wrap the samples in extra bubble wrap or an old gift box (like jewelry comes in) in a padded envelope. Priority boxes are easier (free boxes) but it's 4.95 to ship of course.
I've only been asked about liquids at the PO when sending internationally, and then the clerk was satisfied when I explained they were plastic bottles, well-protected, and not containing alcohol. (They get concerned about flammability precautions, I guess.)