Buyers package says arrived

Good morning, so I have had this happen twice in the past month where I ship out an item and of course it’s always over $150 and USPS says that the item was delivered to their address but the buyer does not have the item they’re claiming that they never received the item and I have been trying to get in contact with eBay and they’re absolute useless. Is there some sort of number or a way to get in contact with him better than trying to use their little chat window for help? Also maybe any suggestions on what I should be doing if this happens I’m shipping out at least 20 to 30 items a week so I have a feeling this will be happening again given the track record. eBay really gets my blood boiling.