Put a recognised address as mailing address. You can put your school address as mailing address or if your house has an recognisable (registered) address then you can put it too. Don’t put addresses where you have never received mails in the past- because there are chances that mails might not reach you. You can put DHL address but you must talk to DHL in advance and let them know that you are putting their address so that they know its yours.
Should I use DHL address as my mailing address?
My brother had opened up a mailbox in the post office in Sundhara and that is how he would receive all his documents from the colleges. However, the i20 is a very important document, so I would recommend asking them to send it through FedEx or DHL and you can go to any FedEx or DHL office in Nepal and get it from there.