Merchants on Business and Enterprise plans who want to receive underpayments (and then credit them to their customers) can choose to do this automatically via a setting in their BitPay merchant dashboard.
Go to https://bitpay.com/dashboard/settings/edit/exceptions, you will see the following screen:
Enable underpayment acceptance by moving the slider to "Yes." Then select the maximum underpayment percentage you're willing to accept on orders.
In the picture above, we've chosen to accept all payments that have received at least 95% of the payment amount we requested at checkout. The underpayment discrepancy can be no greater than 5%. If it is, the underpayment will be refunded to the customer, who can try again later.
You can also set a static price setting. If you don't want to credit orders which were underpaid by $0.50 or more, you can set that preference in this setting.
Here is our FAQ video on how to automatically accept underpayments: