This "feature" is awful. I worked in system analysis and design for years, and it's clear to me that a well-intentioned feature was implemented in a way that does not meet the workflow of a large set of your users.

I can understand having a "canned" set of items that you can choose from, but if you're already populated the details there should be the option after the fact to change the Product/Service without it wiping out the rest of the information on the line.

The system could then prompt you to see if you want to save that particular "Product/Service" to the set of canned items, or not.

In our case, most line items are unique and there's not really any value in saving a predefined set of them, which really makes this a double-whammy of useless and inconvenient.