I would like to know if quickbooks supports Mercantile bank in its bank feeds?

Hello there, ajstewartaa,

You'll want to check on the Connect bank account window if your bank is supported. QuickBooks support a lot of banks. Just in case Mercantile Bank is not yet supported, you can send a request.

Here's how:

1. Click the Still can't find your bank? link.
2. Enter the bank's name and bank we address (URL).
3. Click Request.

For now, I would recommend manually importing your bank transactions into QuickBooks Online. 

You can check this article for the detailed steps: Manually upload transactions into QuickBooks Online.

Stay in touch with me if you need anything else.