Two different ounces
The ounce you weigh food and parcels in is the avoirdupois ounce: exactly 28.349523125 grams, one sixteenth of the international pound. The ounce that gold, silver and platinum are priced in is the troy ounce, which is 31.1034768 grams — about 9.7% heavier. A quoted gold price “per ounce” always means the troy ounce, and treating it as the ordinary one understates the metal by nearly a tenth.
Twelve troy ounces to the troy pound
The troy system compounds the confusion by using twelve ounces to the pound rather than sixteen, so a troy pound (373.24 g) is actually lighter than an avoirdupois pound (453.59 g) despite its heavier ounces. Outside bullion and gemstones the troy system is obsolete, which is why it catches people out when it appears.
Fluid ounces are not on this page
A fluid ounce is a volume, not a weight, and the two are only interchangeable for water and only approximately. A US fluid ounce of water weighs about 29.6 grams; a US fluid ounce of honey weighs about 42 grams. If a recipe asks for fluid ounces, you need a volume conversion and, for solids, the ingredient density.
The kitchen shortcuts
Treating an ounce as 28 grams is 1.2% light; treating 100 g as 3.5 oz is 0.8% heavy. Both are fine for cooking, where a few grams either way is lost in the measuring. For postage brackets, where a rounding can push a parcel into the next price band, use the exact figure.