Contact
One person runs the project and the same person reads the mail. A reply usually follows within a few days.
Write in English or Russian — either works.
What to write about
An error in a calculation or an outdated rate is the most valuable message you can send. Those fixes go first.
A request to delete your account and the data attached to it.
A tool you looked for and did not find.
Questions about where a particular figure on a page came from.