Instead of customers having to individually add line references to every line of their order in turn, why not turn on the Job/Project/Line Reference feature and allow them to add a reference against the first product they add to their cart, this reference then auto populates against any other item added to the cart until changed or they end their session. Speeds up ordering and makes it easier for your customers to breakdown their orders on arrival.

First thing to do is decide what you want to call this line reference (our default is Job/Project/Line Reference) but you can amend this via Store Settings > General > Site Profile > Labels

This label will then be displayed to any customer that you activate this feature for e.g.

To activate this for a particular customer simply go to Customers > Accounts > Select Account > Edit Account scroll down to the 'Other' area and enable the toggle for Show Job/Project/Line Reference > Save.