Not really a problem.
First contact your seller. If the item has not already shipped, he can write on the printed label "Att'n Jianymass' Uncle".
But if it has already been sent, either the postie will drop it in your mailbox* anyway, or your uncle can go to the PO with the delivery slip the postie left and pick it up.
Uncle will have to sign for it, and will need picture ID, like a passport or driver's license for identification.
*If you are going away for a while, the PO will sell you a Hold Mail service which can be for days or for months. However, it doesn't really work well with parcels.
