Contact the USPS and the vendor to see if there are any weather-related issues that could be contributing to the delay. Parts of the country have been hit with severe weather, which is messing things up for everyone. If you cannot get a satisfactory answer, file a claim with USPS, and ask the vendor to also file a claim.
We have had better luck lately with USPS than with UPS or FedEx lately. A few months ago, my daughter ordered contact lenses through a company recommended by her insurer. She opted for "expedited" shipping via FedEx, and paid extra for it. She got a shipping notice and tracking information the day after she placed the order. It took FedEx a week to get the package to our local Post Office. Once the Post Office got the package, we had it the next day. This was the FedEx idea of "expedited" shipping. The next order she put in, she took the free shipping option, via the Post Office all the way. She ordered Sunday night, the package was in the mail system on Monday, and we received it Wednesday. Same company, same size package, same origin location (Columbus, Ohio), same destination address.