Outside of the one ebay uses when you purchase postage from them? UPS/Fedex you'd have to go directly through them for their insurance. USPS also offers its own insurance directly.
Yes, I have used SHIPSAVER insurance company over the years. I first learned of it when it was an Ebay approved app and it coordinates with your Ebay items sold. It is much less expensive than the USPS insurance cost.
I agree, best third party insurer is ShipSAVER, which integrates with ebay. Cheaper, easier, and faster claims than either USPS, UPS, FedEx, or ShipCover.
