Amazon requires poly bags on any unit that could contaminate or be contaminated by other products, as well as all loose items that could be separated during fulfillment. This includes apparel, soft goods, fragile items, and anything with small parts.
The bag must be transparent, have at least 1.5 mil thickness, and be completely sealed. For bags with openings larger than 5 inches, a suffocation warning is required in 10-point or larger font.
FNSKU labels go on the outside of the bag. Placing the label inside the bag or under the product inside the bag fails receiving scanning.
When scoping prep with your 3PL, flag which SKUs require poly bagging in your prep specification. A center that assumes all units need bagging will overcharge. One that skips required bags will cost you more in chargebacks.