I would say that your use of checkboxes is off. There shouldn't be any need to explain that if a check box is ticked then this sentence applies.
Royal mail have got it down to a pretty good art:
http://cdn1.alphr.com/sites/alphr/files/images/blogs/wp-cont...
and earlier:
http://1.bp.blogspot.com/_ZKsGEgLdzMc/S-kztysmnJI/AAAAAAAAAL...
The main issue I have is that there is too much space devoted to telling you stuff you already know. You know the destination address because thats why you have the card. More importantly there isn't a space for a tracking number. (I assume the USPS has some sort of tracking.) surely that's the most important thing?
Dear _______ We missed you,
Your package __________ was:
[]too big []needs payment
[]needs signing for
and is now back at the post office.
[] I will try again AM/PM
[] The package will need collecting
Date_______ Time_______
Notes:
_______________________
_______________________
On the back collection information.
Much more simple, readable and compact. The flow of information is from top to bottom, most important who its addressed to, next why its not been delivered, then when its being delivered (if at all) Date and time of attempt, extra notes.