I faced this situation number of times already.
It is impossible to include all items in one invoice.
In cases like this I issue two invoices and US invoice I usually do with Zero shipping and apply combined shipping cost to Canadian invoice.
The invoice in US dollars has a note that ZERO shipping is because the item(s) will be shipped together with item(s) listed on Canadian invoice.
The Canadian invoice contains a message that the shipping cost is accommodates item(s) invoiced under US billing.
Good luck