Originally Posted by itsme
If you pay a 2.49% "convenience fee" and realize "double miles" for it, then you are paying 1.245 cents per mile purchased this way, right? (If you aren't getting "double miles," only the usual 1 mile/$1 charged, then you are paying 2.49 cpm.) If instead of 2 miles/$1 charged, you get 1.5 miles with the Juniper MC, ought it not come out to 4/3 x 1.245 cpm, or 1.66 cpm or thereabouts? An extra 1.5 months of interest may bring the net cost down a bit more (1.4 cpm?), but that interest income is presumably taxable, so not as sweet as might first appear.
I think that my offline, pen-and-paper analysis leading up to the 1.4 CPM number was fundamentally the same as what you wrote above, with the exception that I forgot to include the tax liability on the interest income (oops!), so maybe push that a bit higher. The numbers definitely look a lot nicer when you're talking about double miles though.