To help give you the flexibility you need, you can split a payment between two billing methods by using the Pay Now button. You can choose how much of your balance you want to pay with your preferred billing method, and then make a second payment with a different billing method.
To do so, go to Settings > Billing & Payments > Pay Now to make the two payments.
Pro tip:
When you use teams to organize your projects, you can assign a different primary billing method for each team.