This might work:
Southwest offers gift cards on their website. You can actually have the "gift card" emailed to you for free. See their webpage for it here:
http://southwest.com/cgi-bin/giftCardCreate
Once you have your $50 gift card, make your reservations. When you get to the page where you need to put your payment info in, there's a section for "Apply Travel Funds". Click the "View and Apply Travel Funds" button. You can enter your gift card info here and apply it towards the cost of the ticket.
Once you've filled in all that info and clicked "view funds". This should show you how much is on the gift card and asks how much of it to apply to your purchase. You can apply any or all of your funds from that card to your reservation.
The website then returns you to the "purchase" screen where you can enter the information from your credit card to pay for the balance.
Hope this works for you!