We are currently evaluating this issue and will update this article when we have more information.
Temporarily mark the 'Send mail to this address' on the preferred address to send the eReceipt and re-mark after the merge is complete. Follow the steps below to globally update records for a Query of records.
1. Create a Query of affected records
2. Navigate to Admin, Global Change Records
3. Click the Include button and select your Query
4. Include the field Preferred Address, Send Mail and select: Operation: Add, Add Checked, and mark the checkbox to Overwrite existing value
5. Verify your selections are correct and accurate
6. Click the Change Now button
7. Save this Global Change to use after the eReceipt merge and reverse the Operation to Replace