Would "Ĉu vi povus sendi ĝin retpoŝte?" also be correct?
Yes, it would.
Be careful, it is poŝto with ^! And “posta” and “rete” in Italian mean post and net.
What is the difference between retpoŝto and retmesaĝo?
Check this lessons tips. Basically:
Retmesaĝo: email message
Retpoŝto: email service
why not "povis"? I know povus is also right, but can't you "could" see as the past simple of to can?
If you have to ask that question about the past the simple past is the wrong tense.
Because send is not in past, that sentance would be "Could you have sent it by email"
is 'Cxu vi sendus gxin per retposxto?' not the same? or would that be 'should you send it by email?'
is "…per la retpoŝto" wrong?