How do I serve papers to take my landlord to court when his listed business address is actually a drop box at a UPS office

I'm trying to get a refund of my security deposit. The landlord followed none of the rules by Colorado law. I got no accounting, no inspection report and in e mail he blatantly lied and kept changing his answer about why I wasn't getting a refund. I tried sending the recommended letter telling him he had 30 days to answer or refund my deposit or I was taking him to small claims. The certified letter I sent couldn't be delivered because it was a drop box. I sent the same letter regular mail and got no response. How do I serve him papers when I can't find him? He seems to be hiding. He has the drop box listed on his web page, BBB listing and his Realtor's license. We called and were told they had no physical office.