There are 2 actual errors in the message
First, Mongolia, though being a close satellite and strongly dependent state, was not a part of the USSR (or Soviet Union), neither at the time when the cover was posted, nor earlier or later. Formally, it was an independent state.
Second, the face value of the stamps and indicia of the cover are not in rubles, they are in mongo, fractional parts of tugrik, the Mongolian currency. So the rate was 150 mongo, or 1.5 tugrik.