Visa Question

Привет! I am having a hard time finding information on what I need to do using my Multi-Entry visa. I'm a US citizen and currently have a valid 3-year Visa which expires in June 2019. I traveled to Russia last year and used a Russian travel company to help with the visa application process. I obtained a private stay invitation letter from my mother in law, and the travel company provided me with a registration card that I took to their office when I arrived in Moscow. I will be returning to Russia next month and will stay with family for 5 days in Moscow, and 5 more days in St. Pete's. Do I need another invitation letter, registration card, or migration card? I’m just really not sure what I need for the second time around, so any information is appreciated. Спасибо!