Why aren't all the items on this order printing on the customs label
If you have 7 items showing when you are printing the customs label for shipping to the states you most likely have your printing output set to thermal. This only allows for up to 14 lines or 7 items of the purchase to be printed. If you have an order where there are more items and you want them all to be printed you are going to have to change the printing to 8.5X11, recreate the order and print the labels again.
Once you send the original manifest to canada post for an order you can't simply change the output as it will still send you the thermal label associated with that order.