Mailing GC (via courier to India) - Thoughts/experiences?

I received my parents GC in the mail recently but they left the US a few months ago. Having said that:

- Is it acceptable to Fedex/UPS the card? I mean is it legal?
- How long after the GC has been issued should my parents come back to the US? Is there a time limit that they can stay abroad after the card has been issued?

- Last and most important question, has anyone here sent the physical card to India via Fedex/UPS? How was your experience? I have heard packages are occasionally opened or 'missing contents', but this may just apply to heavy packages that are of monetary value and not documents? Any thoughts, experiences?