Post office wants me to send in a box
We do occasionally get reports of post offices wanting people to send our kits via an official 'cremation box.' This is not the regulation, which is to send ashes in a double sift proof container signature express with cremation stickers.
However, you will not 'win' an argument with your post office as the post master has final say. You could try shipping in a small priority box (but express) to address the 'must be in a box' comments, but we also suggest trying a different post office. We experience this issue with about 3-5% of post offices.