Just because you have the receipt dosen't mean that you are off the hook. You opted out of getting a tracking number for your parcel. Yes, I understand getting one is expensive but that is the business decision you took. When you decided to begin selling online and using the postal service to get your items to the correct destination, you take the risk of using the postal system. While most items get delivered, some do not. That's the risk you take. And when the risk turns sour, you chalk the instance up as a business loss and move on.
If you're still upset, put yourself in the buyers shoes. You pay for the shipping and the item, and are excited to receive it. But after some time has passed, you don't get anything. You message the seller, but the seller's response is "I mailed it, I have a receipt, sorry but not sorry". No. That's not how business works.