Unique Mailing Situation. What Would You Do?

Recently i sold $190.00 worth of parts, that was paid in full, thru Pay Pal, Family or Friends.
The same day, i mailed out the parts to him, at the address that he gave me.
A couple of days later, i got a e mail, that the small, flat rate, USPS box is being returned to me as "insufficient" address.
Again, i wrote down the address as it was given to me.
I didn't make any error in addressing it.
I wrote him an e mail, and he didn't respond for a couple of days.
Then i left a message on his voice mail.
Still no response.
I GOOGLED the address and come to find out, it's a UPS store.
So i have no idea why the USPS couldn't deliver the package to that address.
Voice mail, and more e mails to him asking to give me another address to mail to.
Still no response from him at all.
Now what would you do?
I got his money, and i still also have the parts.
Do i just keep my mouth shut, and let him make contact with me, and just keep the parts?
Nothing like this has ever happened to me, in probably selling parts for 20 + years.
Jim V.
hemi71x