Consider renting a car in Moscow for a day or several days. Car sharing is convenient for short trips, but a trip to St. Petersburg can be more expensive due to per-minute pricing. A traditional car rental offers a fixed price, which is very important. Transparent terms and conditions and the absence of hidden fees can be found here
https://getrentacar.com/en-US/russia/moscow The price includes insurance and mileage. You can both car and rent in one place, without being tied to a schedule, but check return restrictions in another city.