I vaguely remember getting tabs in an envelope with a birthday card or something. MDPV I got with a bracelet. Customs don't "hard check" every single package; they rely a lot on declarations and random checks. So so long as the thing being sent with the drugs is plausibly mailed, you're generally safe.

(e.g. The package with the drugs in it would say it has a "goth novelty bracelet" in it, but it ALSO has a vial or a bunch of baggies)