For clarification: Does the "expiration date" of travel funds indicate the date you have to book...
I've worked in aviation for over 25 yrs always in the mtc field, worked for delta and southwest and the navy,...
Your flight must begin and end before the expiration date of the travel funds - you may extend the travel date by 6 months for a $75 charge. -the $75 charge is not taken from a credit card but deducted from your travel funds if you request to extend travel funds.
Here is a link to some information that may be of help to you on Southwest.com
hope this helps.
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