Can I use a qualified electronic signature certificate issued in the name of a natural person, even if I open a drawer for a legal entity or self-employed natural person?

Yes, a certificate issued in the name of a natural person is appropriate. The only condition is that the person opening the mailbox is a representative of a legal entity. A qualified electronic signature certificate is used to sign documents and identify the user. An electronic signature is equivalent to a physical signature regardless of how the certificate was issued. It is only important that it is issued by a recognized certifier in accordance with the prescribed procedure.