I have been conducting research using the fruit fly Drosophila melanogaster since I started my PhD in 1982. In that time I have imported countless consignments of fly strains through the post and by courier (such as Federal Express). On only one occasion can I recall having difficulty getting them through customs - a box of female-sterile mutants from France got stuck, and was in a frightful state when they arrived (this was during my PhD).
The general practise is for the sender to affix one of those green customs tags, asserting that the contents are a gift, of no commercial value (typically $1 may be quoted), that they are live insects, but not an agruicultural pest or a vector of disease. So for over 26 years as a practising Drosophila geneticist, I've had but one case where this has presented a problem.