Unfortunately without a signature you're likely going to end up out of pocket in this situation. You need to make sure everything you send has signed for delivery on ebay for this very reason.
Ebay will almost always side with the buyer in this kind of dispute, in that respect it's a haven for people who want free stuff.
Advise that as far as you're concerned it's delivered and there's nothing more you can do, maybe the buyer will chalk it up to experience, more likely is that the buyer will open a paypal/ebay dispute, ebay will spend a week "investigating" it, and then you'll likely need to refund the buyer their $150.
As you have some proof of postage, you may get lucky and ebay will find in your favour, fingers crossed for you but without insurance you could have shipped a brick so the likelihood is, they wont.
Sorry I don't have better news, but I sell often on ebay, and unfortunately, that's how the cookie crumbles, some people order stuff on ebay specifically looking for people who don't use signed for delivery so they can run this scam.
Best of luck!