Not sure what type of "receipt" you received but there are several types that have a charge.
Certified Mail
Return Receipt can be added to confirm delivery. Delivery information is available by hard copy or online.
$1.85 extra or $1.35
Insured Mail
Provides coverage against loss or damage up to $5,000.
Fee based on value of item. Starts at $1.35
Signature Confirmation
Provides the date and time of delivery or attempted delivery and the name of the person who signed for the item. You can request a hard copy of the signature. Delivery information is available by hard copy, online, or by calling 1·800·222·1811.
$1.90, $1.35 when you print Priority Mail labels from
Click-N-Ship.