i will be arriving to StP on the 7/09, is it possible to pay with usd for the taxi or they accept only rub ?
Officially, at least, I believe you must have ₽.
There are plenty of bancomats at LED where you can withdraw what you need.
what about exchange at the airport ?
Exchange at the airport would probably give the worst rate possible. Official taxi desk accepts credit cards and give fixed rate for the ride. Or take Uber...
You may exchange a minimum amount as an alternate or try Uber or Yandex apps for taxi. I have booked via them and found that they are not only cheap but also I do not need to exchange a single word with driver.
Why would you want to pay in $ US ? Whenever I go 'abroad' I always exchange to local currency before my trip.
Here is a cut and paste from a post a few days ago from user Another T:
Rubles: '2000, 1500 or 1050. Luxury, standard and economy, respectively."
Recently, Uber cost 487 rubles for the same trip.