If you are merging in related lists of records and need specific control over the order of line items that cannot be set in Salesforce, you can use specific queries with Rowburner. This allows you to dictate which types of records are merged into which rows by using different filters on the associated SOQL queries, and Rowburner will remove any rows that do not apply to a particular merge.

In this example, we merge all Recurring products first, followed by all One-Time products: