Effective with Version 2019, Build 914, it is possible to show Credits in email remittance notifications.
If integrating with QuickBooks: refer to QuickBooks Credits in Email
If importing an Excel or CSV file:
1. In the Email Setup, be sure to choose to Consolidate the transactions--
Continue through the wizard until you can click 'Finish'.
2. You will map the Invoice data with the mapping header "Check_Stub_Invoice#"
3. The resulting email will look like this: