Short answer
HubSpot's KB: when you import line items or update them via the API, the deal amount isn't updated. The Line items card shows up to 200 line items. Custom items outside the product library need the Create custom line items permission.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
In reports, line item currency matches the deal amount's currency.
3. Related
Copy line items and the product library.