I knew of something like this but still signed on with them for a 10k alaska promo they had a while back (year or so plus?)
BUT they were never able to tell me the exact amount which I WAS wondering about enough to contact CS and ask. they told me the tiny tax can fluxuate and though it was not much money I just needed to confirm what it was and then I was ok with it.
One thing I did learn on the same phone call was that you MUST make at LEAST $1 per calls in any month to keep your FF gig going with the sprint service or else you may forfiet all miles from then on! SO, at least that CSR was helpful with this info!
ALWAYS read ALL fine print. They know we dont so that's where they stick it in.
Anyone recall the old legend about how Van Halen had a small clause in their contracts stating they could trash back stage and NOT get billed anything for it if there were ever any brown M&M's found in their food cache back there? Sort of the same thing. Statiums would try to bill them or sue them for damages would get no where because it was in the fine print. NO brown M&M's allowed/
i dunno if it was true but it is a great story that relates here anyway.

MM