I don't brew but have friends who do, so I think this is a great idea, especially if you add tiers or every-other-month options.
For comparison, a super high gravity Belgian black will run about $60, including lots of Belgian Candi Syrup. Your average nut brown will run about $20.
$15 to ship is a killer.
I agree 15 dollars is high but shipping 10 or so pounds is about that much.
Otherwise, cool service!