Unless it specifically says Restricted Delivery then they can't stop anyone over 18 with the Slip to pick it up. You can always send it with her signed. Look on the back of the Slip, left side and check they box authorizing her to do so.
Also, they signature thing is a new "revenue creator" that they - management- has instituted for us. It's a pain in the ass, trust me. I deliver them daily. All you have to do is put a note on your mailbox. Tell him to scan delivered, press 8 and sign the damn thing. Again, something I do daily. Pink slips suck and I know most of the people on my route. They never get a pink slip, just a package. =)
I'm not a feather plucker or a feather plucker's son but I'll pluck a feathered pheasant until the feather plucker comes.