A spell that causes items to assemble themselves into a trunk.
References from the canon
Tonks used this spell to help Harry pack his school trunk when he was leaving the Dursleys' house. She said she wasn't very good at housework spells and that her mother used to be able to even make the socks fold themselves up together. Tonks' version was a bit messier, but it certainly got the job done quickly (OP3).
It's quite likely that "Pack!" is not the incantation (it sure doesn't sound like an incantation), and Tonks was just using a nonverbal spell while conversing with Harry.