Yes, there is something wrong here. On Marti Michell's website, the Sashing Stars set is $17.98 and the Long Skinny set is $15.98. So you shouldn't be charging +$10 for the Long Skinny. I think you mean to say that it's an additional $10 to get BOTH sets.
SWMBOI thought this same thing the last time this was offered. I thought I was getting both sets when I did the $10 add. No, I only received the long skinny set. I did email the help group and verify that it was meant as instead, not add. I did point out to them then about the price difference on the Marti Michelle site, so I was surprised there was not more clarification this time around.