I'll explain this in terms of theory instead of "do this"
Items have a "grade" column, this "grade" is how you assign the drops ( you should know this )
Boxes are items that behave like mobs...the "drop list" for boxes is in the itemcreate.ini file.
In the database, find a "box" and look at it's ReqVg column. The number in the ReqVg column is the Itemcreate_## in the itemcreate.ini
[ItemCreate_##]
Item# Droprate
## is the number in the boxes ReqVg column
All of this can be found in the how to setup Shaiya pserv pdf somewhere in the sticky area

Read through it. It's got a huge amount of good info in there all in one place.
Happy gaming