Quite a few People I have bought items from on eBay have used USPS Flat Rate Priority Mail Boxes or Envelopes with Delivery Confirmation.
You don't waste time scrounging around for a container to put the item in and as long as the item fits and is below the max weight you know what the shipping cost will be.

When I sold the little Tools I used to make I used USPS First Class with Delivery Confirmation. That ended up being an actual cost between $2.50-$3.00 shipping for most items.

If I had a something that I knew would cost more I packaged it without sealing it an took it to the Post Office and ask that to weigh it at tell Me some of the ways it could be shipped. In My case the Post Office is down the Street from Me.

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84 300D, 82 Volvo 244Gl Diesel