Hopefully most reasonably intelligent folk spot that! The shipping prices certainly are not hidden for each seller. Though it is worth noting since each seller sets their own shipping prices they can vary widely between sellers.
You are right too, it is an interesting game to make sure you get the combination of cards and sellers right to if you want to minimse shipping/total cost.
Just yesterday, I actually purchase a card from one seller for 0.95c when I could have got it off another for 0.50c. Adding it to the cheaper seller would have bumped me up a shipping bracket and an extra $1.40 while it made no difference on shipping an the pricier guy. So pay twice as much for the card but save nearly $1 overall.