Starbucks Moral Dilemma

So in the beginning of January Starbucks sent out a card in the newspaper that entitles the bearer to a $2 tea drink after 2 pm everyday though the end of February. Only thing is due to a glitch in their system this card has turned into a free tea drink after 2 pm everyday through the end of February card. Is it moral to keep using it? Should I send Starbucks a check for $6 (I've had 3 freebies so far)? Is Starbucks a big heartless corporation that couldn't care less if one college kid is cashing in on their dime? Am I allowed to stop in every Starbucks I pass after 2 and cash in? Feel free to weigh in.