An easy way to do this:
Go to your local liquor store and pick up a case that was used for wine. These boxes have individual slots for each bottle of wine and they are deeper than beer cases. Use the interior slots for your bottles. Pack newspaper into the bottom of each slot and fill the external slots all the way to the top. place each labeled bottle in a bag and slide into alternating slots and then pack more newspaper on top of them. I have done this many times and it works great and you are not spending any money on packaging materials.
AS mentioned it is against the law to use the USPS. I use FedEx, have an online account, print my own label and drop off, I've never been asked what is in the box.