Don't this that is correct. The email I got has these rules:

OFFER DETAILS:
Discount Amount: 15%
Booking period: Book through 10/15/02
Travel period: 7/31/02-10/31/02
Eligible Booking Class: First, Business, Economy
(F,A,C,D,Y,B,M,H,Q,V,W)
Maximum number of passengers per itinerary: 1

NOTE: Not all flights are eligible for an electronic certificate
discount. In some instances you may find flights that are not
electronic certificate eligible, because they have already been
discounted and are less expensive than the electronic certificate
eligible flights.

IMPORTANT NOTICE:
We are not yet able to redeem paper certificates, coupons or vouchers
at united.com. Please contact customer support at 1-800-589-5582 with
any questions concerning paper or electronic certificates.

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Land O' Lakes