What's the difference between a poly bag, a garment bag and a mailer?

A poly bag is a lightweight single-layer bag (20-40 microns) used to protect individual garments through the supply chain. A garment bag is a poly bag specified for apparel: sized to the folded garment, with resealable or permanent adhesive closures. A mailer is a heavier bag (60-micron baseline) engineered to survive the courier network as outer transit packaging. Garment bags protect the piece; mailers carry the brand to the doorstep.