I frequently buy upgrade coupons from United, either at the ticket counter or the gate. Sometimes they are a bit strange if you do it at the gate (scolding---you should have done this before now), and one time at Orlando they told me to go up to the Red Carpet Club (this 10 minutes before flight time (?)) to purchase the certs. Sorry gate agents--I don't want to overinvest in certificates if I may not be able to use them.