According to the U.S. Post Office website, “Certified Mail service provides the sender with a mailing receipt and, upon request, electronic verification that an article was delivered or that a delivery attempt was made.” The idea is for the sender to be able to say that he did mail the requested material. The responsibility is not on you to ensure that the mail gets to you. Receiving certified mail is not much different than receiving regular mail. You have a few options when certified mail is delivered to you.
Sign the green card on the envelope or package when the postal carrier delivers it to you. This serves as a receipt to the sender that you did, indeed, receive the letter or package that was sent.
Go to the post office nearest where you live, and pick up the certified piece of mail if you were not home to receive it. Take the card notifying you that you have certified mail to pick up that is left by the letter carrier in your mailbox with you to the post office.
Refuse to sign the green card and accept delivery if you do not want to accept the certified mailing. You can also just not go to the post office to pick the letter or package up. It will be returned to the sender eventually.