An electronic letter seems to be okay at the moment, due to Covid issues.
Electronics payslips should NOT be stamped anyway, so that's not an issue. They must however be verified in a letter from the employer, but it sounds like you already have that, so that's good.
My question is how do I submit those documents? Do I need to print them, scan them and upload the scanned copies, or is it OK to just use the electronic copies I already have?
I would just upload the electronic copies.
Since they are electronic anyway and do not need to be stamped/signed in person, and your employer letter is already in the required PDF format for uploading, there should be no need to print and then scan them again.