I do agree with the weight issue and you need to ensure that the package sent to you did weigh something along the lines of what it should contain.
Getting written statements is a vain cause, because the person writing it is somewhat accepting responsibility - or so he thinks.
Now, if the box did weigh-in correctly and the package was delivered to your neighbor and he did not "refuse it" or note the package was "tampered", then UPS, etc.. are probably off the hook - unfortunately. Because they delivered. But, UPS normally have a period within which you can complain....
To me the issue is in the delivery proof and what proof is deemed valid according to the applicable contract. You sent the mac for repairs based on your AppleCare contract. Apple (or their repair man) chose UPS for return delivery.
Stick tight as hell to the AppleCare contract, that you did not choose UPS for return delivery, you prefered another delivery service. Make Apple responsible of honoring their AppleCare Contract. The repair was AppleCare, you talk to apple.
Just like if you are sick of eating some peas you go for the manufacturer not the retail shop.
If you sue/attack, Apple is the entity. Let them sort it out with their repair service and their carrier. You have no say in who is used (for repairs and/or return delivery), you cannot assume their errors. Again, you would have chosen another delivery service if Apple had let you choose.
lots of luck
