It depends on the term stated on the coupon itself. Some require customer to pay the tax, most don't. If the coupon doesn't mention who pays taxes then I would redeem it at SDM. They usually entered as 2002 so it's totally free. WM's policy is customer pay taxes if it's not clearly state "taxes included in face value of coupon".
Next time you're there ask them to refund you the taxes that they incorrectly charged you. You don't have to pay taxes on Jergens / Minions.