Shiptheory is able to pull through your specified amount of boxes from your invoice in Xero.
This can be done by typing "boxes = x" (x being the number of boxes required) into the notes section of the invoice. One important thing to note is that the text string is not case sensitive and does not need spaces. For example, boxes=2 will work as well as Boxes = 2.
See below for an example:
One important thing to note is that updating the notes section in Xero doesn't count as an "update", meaning that you'll need to fill in this section and then update the order for the amount of boxes to be pulled through to Shiptheory.