Most apparel moves through warehouses in polybags. They keep shirts clean, let a picker read the size sticker without opening anything, and stop prints rubbing against each other. Common features are a self-seal adhesive flap, small vent holes to let air out, and a printed suffocation warning.
Suffocation warnings are required by some US states and by many retail and marketplace buyers once a bag passes a certain size, so check the rules where you sell. Bags are commonly made from low-density polyethylene, and recycled-content and compostable films are also available, each with its own trade-offs.
Ask whether garments will reach you bagged and labeled by size, whether the bags can carry your branding or a size sticker, and how they will be packed into mailers. See packaging and unboxing.