None of which is relevant for a $5 box of fibre bars, which fall below the de minimis threshold.

Please cite the de minimis threshold you are referencing. Here is a basic 67 page overview by the IRS on this topic [1] which does not indicate any uniform generally accepted de minimis threshold for arbitrary expenses, so you need to cite specific laws, regulations, case law, or generally accepted accounting advice to support your statement.

[1] https://www.irs.gov/pub/irs-pdf/p5137.pdf