Change the way you are thinking about this. Instead of you having to prove you didn't receive it, which is basically impossible, tell them to prove you did receive it. If they didn't have a tracking number, and didn't ship it asking for a delivery receipt, they can't prove you got it. You have proof they had control of the engine, the repair invoice. They need to be able to prove they took reasonable care to get it back to you. Personally, I think it's sloppy on their part to ship things without being able track and confirm delivery. Without this proof, they're probably going to have so suck it up and send you a replacement. Be sure to send written notice to the shipper they used, to your credit card company, if you paid for the repairs by CC, and to the manufacturer stating that you never received the engine. You'll need to be able to prove that you notified them and tried to resolve this.