No one is reqd to sign for a certified letter. if he is trying to serve you that way, he will have to have someone else serve you. He may have a friend do it and save process serving fees.
ORS 46.445(3) allows for service of a small claims complaint by certified mail. If you don't answer the complaint and the defendant then applies for a default, there will be no one to speak on your behalf and the court may grant the judgment against you, after which it might be you stuck spending money and time on the process. If not, the sheriffs office will serve you for $36, or he can have a friend do it. Not a huge barrier to litigation.
As for the demand letter, there is no requirement that it be served by certified mail. It is only required that the plaintiff attempt to collect the debt before suing. You refusing to accept mail just makes you look like a jerk, if not outright culpable.
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