How to add more than 10 items to USPS international customs form

We got an order for 26 items from the US to Canada. However when we print our customs forms only 10 items will show up. How can we get everything ordered to be included on our USPS custom forms? Talked to shopify chat and they said USPS limited it to only 10 line items. Never had this issue with shiprush in the past, so I find that answer interesting. Has anyone else ran into this issue or found a solution?

Thanks!