How do they handle it when you get your inventory sent back? I.e. you send them 100 pieces of X and after three months you tell them to send them back to you. Do they just take 100 pieces of X out of their general inventory, or do they know which ones were yours? Because otherwise it seems like an obvious scam where you could send in trash and get real items back.
RFID is a possible solution. Tags are now about five cents, and likely to drop further in price with volume. Newer tag designs can be sewn in the seams of garments and shoes.