RE: Import Product Line Items to Opportunity
Hi Dan,
Are you using Sales Professional?
The import template for Opportunity Product in Sales Professional does have only a few fields and the Opportunity column is missing.
When using Sales Professional, we should export the view as a template instead of using the OOTB template.
Open Advanced Find, search for Opportunity Product, then Edit Columns.

Add Columns and select all fields.

Click Results, then export it.

In Excel, drag out the hidden columns A, B, and C. Only when the A, B, and C columns of a row of data are empty, this row will be regarded as creating a new record. If the A, B, and C columns of a row of data contain values, then this row will be regarded as updating an existing record instead of creating a new record.
