If you read the Terms & Conditions, it says there is no purchase necessary. According to them:
"No Purchase, Ticket or On-Line Check-In Necessary to Enter: You may also enter the sweepstakes, by hand-printing your complete name and address, including zip code, phone number and Delta SkyMiles number (SkyMiles number is optional) on a 3" x 5" piece of paper. Mail your entry in a hand-addressed (#10) envelope to: Delta "Mini Cooper" Sweepstakes, P.O. Box 6721, West Caldwell, NJ 07007-6721. Entries must be postmarked by January 2, 2004 and received by January 16, 2004. Sponsor not responsible for lost/late/misdirected entries or mail.
You may enter as often as you wish via mail but limit one entry per outer mailing envelope. Sponsor not responsible for lost/late/misdirected entries or computer malfunctions. Entrants may be automatically enrolled into Delta's Email program (no purchase necessary to receive the Email program and entrants may cancel the Email program at any time). "
Time to bribe someone to handwrite all the cards....