Can you send certified mail to a UPS Store box?

Yes, but the UPS Store employee signs for it, meaning you have no proof that the intended recipient of the letter actually received it. You only have proof that it was put in the recipient's box. And since the UPS Store is private, even though the box holders have to sign for their mail, you cannot access the signatures showing they picked up their mail without a subpeona. Not much better than just sending it regular mail, if you ask me!