When you are first setup as a vendor, we use the payment details you provided on your sign-up form to create a Bill.com account that uses ACH payment. At the same time, we send you an invite to create a Bill.com account so you can see invoices, payments, credits, etc.
Once that account is created and you connect with us, you become responsible for setting the payment details including re-entering the payment details for ACH if you want to continue using that. If you don't re-enter the details, it will either send you a check or a Virtual Card. Virtual Cards include fees to Bill.com for their use so Bill.com tends to default to using them.
To fix this, login to your Bill.com account and go to your settings. Update your payment preferences to use ACH.