I'd really enjoy seeing something like that in place. Makes it unnecessary to have to type in /mail send (message) separately if the person who is receiving the gift is offline and miss out on the context.
Example: Gift from adam5979: "Here's the pickaxe you ordered."
Of course, maybe implement it like this:
After putting in the items into the gift chest/inventory, then a message will pop up, asking if you want to write a message along with the gift before sending it. Then type either /yes and write the message, or /no. After that, it'll send out the gift.