It is rather interesting to see some of the media hyperventilate about the cutoff to Bulgaria, when in fact Bulgaria is extremely well positioned to switch supply of its meager gas import needs (3% of its energy usage, plus some industrial uses.)
Clearly, however, the Bulgarians (boosted by good past planning) are in no mood to countenance a meaningless threat, and are telling Pooty to take his gas and put it where the sun don’t shine.
I’m sure global gas prices will ding the Bulgarians just like everyone else importing gas, but, THAT is something the US can do quite a bit about, if Biden can be forced to do it.
Serbia, on the other hand, is heavily dependent on Russian gas. And that gas transits Bulgaria by contract, but Russia has already altered the terms of the deal.