What documents do you need to provide to a foreign client and how to provide services to non-residents of the Russian Federation in general?

Question

Documentation

What accounting documents do you need to provide to foreign clients in the event of the sale and development of software (when selling through PayPal, for example)?
I know about Invoice. How to form it correctly? Is there a service that does this? You need to send the original or a scan is enough.
What other documents will be needed?
How to develop for foreigners? Is writing TK standard or is it possible to negotiate verbally (maybe someone has personal experience)?
Well, the most insidious question. What is "cerificate of residency", context below

By a cerificate of residency i mean a document from your tax authority
that you pay taxes in your country. This is important for my tax authority.

Answer 1

What is a "certificate of residency"

We are talking about a document confirming the status of a tax resident of the Russian Federation (the so-called certificate of residence). It is needed in order to avoid double taxation on your income. What is it and how to get such a document, see the information on the website of the Federal Tax Service www.nalog.ru/rn77/fl/interest/dual_nationality/status