Hi
If the requirement is to send the dunning notice to ship to party, you can use the field Dunning Recipient under the correspondence tab of the customer master. This will send the correspondence accordingly, even though the dunning is on the payer. However, in your case, you have multiple ship to party for the same payer. So use the concept of Grouping Key for the same. Populate the Ship to Party on accounting documents of your SD Invoices in a field. There after add this field as a grouping key field and use BTE 1040 to read the correspondence details for ship to party.
Regards
Sanil Bhandari