Or buy $3600 of tickets at southwest.com. If you have a Southwest Signature Visa card, that should be the only card you use at southwest.com. Note that the Classic card does not give double earnings at southwest.com. The difference in earning pays for the extra $20 in fees if you spend over ~$1500 per year at southwest.com.
The $39 card also does not give you 2 credits upon renewal.