How do I import deposits via an .iff file?

Hi smile3. At this time you cannot import an IIF file into QuickBooks Online. However, you can use the CSV or QBO format available with most online banking providers.

I highly recommend you follow our article on Importing CSV files if you plan to do so as it is very detailed and touches on all the things you need to know including proper formatting. 

If there's anything else I can do for you, just let me know I'm happy to help.