I thought it was going to be hard to find, since this was an arcade game from my childhood... But here's one article from Neogaf.
If you Google "Double dragon 3 arcade insert coins", there are reddit articles, forums talking about this, and even the Wikipedia article talks about this being one of the first commercial games to have in-game micro transactions.
"The U.S. version also features item shops where players could use additional credits to purchase in-game items such as weapons, a
dditional moves and new playable characters in one of the earliest forms of microtransactions in a video game, although this system would end up being removed in the later-released Japanese version..."
Also, not defending Bethesda's practice, but Horse armor also wasn't their first microtransaction for oblivion....
They also had themes and stuff on the Xbox store, and literally told people that these types of things were going to be released.
To be fair - I didn't buy oblivion, a friend of mine had it for Xbox, and I went and sailed the kazaa seas and downloaded the base game + all the DLCs without having to pay micro$oft's ransom. Only pointing out that we knew well before horse armor that gamers will open their wallets for this.