We recommend using regular mailhttps://www.pochta.ru/support/letters/letter. Kindly avoid using registered or expedited mail, as well as any other form of it. Don't be worried that the delivery is going to take too long because only the date of sending the consent matters (it has to be sent not later than in 5 calendar days from the date of receipt of the consent form). The date is verified by the postmark on the letter.