Surely, if you are the one dispatching something, then you are the one who needs a signature (receipt) from the carrier to say they have taken it? As to signing something yourself without knowing what it was, well, what can I say?
Is it common for recipients to pay? Surely the sender normally expects tp pay the carrier.
If I send anything by post (yes, I still occasionally use snail mail) I generally put a stamp on it, rather than leaving it for the recipient to pay.
And if I order stuff online, say from Amzon, or LoveHoney, or Luxury-Sin, or whatever, the seller adds a bit on to my bill, so that THEY can pay the carrier.