You could probably use the free box from the post office if you wrap it in brown paper, and you wrap it correctly (shipping tape, no string).
Be very careful about the size of any box you use, wherever it comes from. If any dimension exceeds 12", it becomes a dimensional weight shipment and you pay based upon volume, which is not scenario "A" at all.
The "C" shops all specify at least one dimension which is easily determined to be more than 12", but they still often miss it, and sometimes they are so careless when measuring, that the dimensions are inflated by 1" in each direction. When that happens, I make sure to mention it in the report because the fee charged for mailing it will not be what the MSC editor expects.
Shopping Southeast Pennsylvania, Delaware above the canal, and South Jersey since 2008