The best non-core feature I ever added to my SaaS service remains live import/export to Google Sheets. My customers are not finance people, but they were using

The best non-core feature I ever added to my SaaS service remains live import/export to Google Sheets. My customers are not finance people, but they were using ad-hoc spreadsheets extensively and most of them through G suite. By implementing bidirectional sync for both master data and outputs they have the best of both worlds. The Google Drive and Sheets APIs (and especially the on-change push notification callbacks) are not a pretty sight but once the twisty maze of nested data structures, webhooks and OAUTH2 and what-have-you were successfully navigated we got an efficient, complete and stable solution that everyone's happy with.