(a) Mail article. “Mail article” means any posted parcel, packet, package, envelope, letter, aerogramme, box, card, or similar article or container, or any contents thereof, which is transmitted in mail subject to customs examination.

(b) Letter class mail. “Letter class mail” means any mail article, including packages, post cards, and aerogrammes, mailed at the letter rate or equivalent class or category of postage.

(c) Sealed letter class mail. “Sealed letter class mail” means letter class mail sealed against postal inspection by the sender.