Use Apsona Document Merge with Sublists and configure an Account-level report as the top-level data source and a separate Payment-level report as the sublist data source.
The top-level report should contain Account recipient and address fields, along with the required campaign filters. The Payment sublist report should include the Payment fields to display and the Account ID for linking. Configure the Merge Action to link both reports using Account ID, then use Account merge fields for the letter and TableStart/TableEnd fields to display Payment details in a Word table.
Opportunity fields can be used for filtering and linkage where required without including Opportunity-level details in the top-level report. Ensure the top-level report returns one row per Account to avoid generating duplicate acknowledgment letters.