the.com/envelopes
Paper that keeps your secrets until someone with a thumb decides otherwise.
means Flat folded paper or plastic sleeves that enclose letters, documents, or cash for sending or storage.
from From French enveloppe, to wrap or enfold; the gummed flap arrived with cheap postage in the 1840s, when mailing finally went mainstream.
Pricing mathPushing the envelope is aviation slang, not stationery.
Window trickGlassine windows let addresses show without retyping them.
Self-seal eraLickable gum is fading; allergies and taste killed it.