According to Wiki "invoice" does not exist in Russia. The closest thing to it is called "счёт на оплату" but it's not a common document.
Receipt is called "кассовый чек" if it's printed on a cash register. If it's hand written it would be called "квитанция"

I guess the only official document you could make and that could work work in Russia for you clients is "кассовый чек", because everything else supposed to be stamped with an official stamp of a company, which I doubt you even have.