Anyone have problems with the postal service delivering Ebay packages

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I mailed a package to a customer two weeks ago. The tracking shows the post office receive it. I got a nasty email from customer saying she will report me to Ebay because she has not receive her package. I called the post office. The package is magically delivered the next day. Friday and Monday I mailed a package, here it is a couple days later my tracking number does not show it was checked in at the post office. I took both packages to the post office and gave it to the clerk. Anyone have any ideas how I can protect myself? I put in my Ebay listing insurance is optional and I am not responsible for lost packages. My main concern is if the post office is only showing I print the label then it's going to appear as if I did not mail the package. I am calling the post office to complain tomorrow. I thought about using UPS, but I am not sure if they are better. I am desperate for ideas. I don't want any negative ratings.