There is only one way to do this on our site with standard payment options if you use an Obsessed Garage Gift Card.
Use your first method of payment (Visa Gift Card, Debit Card, etc.) to purchase an OG Gift Card. Then fill up your cart as usual and checkout with that gift card code FIRST, then use your standard payment method (credit card, PayPal, etc.) last. You can do this with as many Obsessed Garage Gift Cards as you need, always use them first.