Here is what you need to do to define a default business bank account and credit card for bill payments in QBO that come over from Ivy.

1: Go to your account settings in Ivy and click into the Quickbooks Tab
2: Scroll Down to the Bill Pay Accounts section
3: Choose a default bank account (for checks) and default credit card to be used. (These options in the drop down are pulling from your QBO account) 

Now when you indicate a payment to the vendor on a PO, the bill payment in QBO will use the default you chose. (in the event that you use a card or bank account that is not set as your default - you will need to update that specific payment method on QBO)