according to the web site they do not allow that...
so there is some fine print to the whole ...... internet only specials...key word here seems to be "ticketless travel"
Internet Specials:
21-day advance purchase
Travel August 21, 2001 through January 11, 2002, except for November 20, 21, 25, and 26, December 21, 22, 28-30,2001, and January 1, 2002.
Fare is only available for travel purchased through the Southwest Airlines web site.
Ticketless travel must be purchased at the time of booking and no later than 12:00 midnight Pacific Time August 2, 2001.
Ticketless Travel may be purchased online using either a credit card or by utilizing funds from a previously unused or cancelled Ticketless Travel reservation(s). Funds from up to four cancelled or unused Ticketless Travel reservation records can be combined to purchase a new Ticketless Travel reservation record. Southwest Airlines travel credit (e.g., travel voucher, gift certificate, discount coupon) may not be used as forms of payment for Ticketless Travel Online.