Don't know how reliable the information is, but a couple IOP's have emailed me that they have much higher U.S. Customs seizure rates on stuff they send with a signature requested upon delivery. If you want tracking numbers, I suggest sticking with U.S. OP's. If you use IOP's just figure that you're going to lose 20-25% of your orders and factor that into the price/purchase decison.
Also, I don't think customs typically sends letters after most "seizures," as I've never received one in five years of dealing with IOP's and experiencing the 20-25% lost order rate cited above. Strangely, I've had several packages opened by customs, resealed with that yellow-green tape and shipped on even though they were clearly filled with blister packs.