redwood, The article is interesting, but the only "advertising" they show for US is encased postage, which is a rather different issue. I'm still not seeing the "Products advertised" on 3c stamps... Not saying it never happened, but doesn't seem to be as prevalent as that article claims.
The Match & Medicine stamps were essentially advertising; one of the selling points was that as long as they stated US Internal Revenue (usually abbreviated Inter Rev) and had a face value equal to the tax rate (one cent per 25 cents retail) anything else was fine.
I don't disagree with that at all. Love the M&M, but that's not what this article is referring to. M&M designs were allowed by the government to expedite the process of getting tax on products at the onset of the civil war. The rush to do it, allowed the companies to create their own dies (hence "Private Die"), subject to approval by the government. But I suspect, you knew this already.