Also worth noting is that, if when following the steps to reproduce that I listed above, if in step 2 you add "Sumatra" (top row, third from the left) to the cart instead of "Colombia" and then apply the coupon and attempt checkout, you get no error. This is very likely due to the fact that "Sumatra" is priced at $14.00 even, whereas "Colombia" is priced at $14.25. This lends further credence to my rounding theory.
Open to all suggestions on this - hackish or otherwise :)
Cheers!