Cosmetics can be sold by weight or volume, it just depends on the type of product. If the product is a liquid (like lotion or shampoo) it should be labeled by volume. If the product is a solid or semi-solid (like a body butter) then it should be measured by weight. Of course, I know with some products it can be hard to determine if it is liquid or solid - if in doubt go by weight - that's always more accurate.
But you need to make it clear to the consumer that when you write 4 oz. on your container - whether you mean weight or volume.
So to help with that the FDA has some rules. If you are selling by weight the term "Net Weight" "net wt" or "net mass" needs to be on the package. If you are selling by volume you can use "net contents" but you don't have to put anything at all.
But when you are listing the quantity, if it's by volume, you need to use fluid ounces or "fl oz" to differentiate it from weight ounces.
And of course you should also put the metric equivalent on there. It's not required by the FDA but it is by the Fair Packaging and Labeling Act. So that would be in grams for products by weight or milliliters for products by volume.
You can of course find all of this out at the FDA website here
http://www.accessdata.fda.gov/scripts/c ... ?fr=701.13But Marie Gales book on Soap & Cosmetic Labeling is really great at explaining all those Regs in plain english. AND she just updated it!
http://www.brambleberry.com/Soap-And-Co ... P3663.aspxI know this is a super confusing part of selling your own products - so I hope this helps!
~Amber