ok 1 - H1 valid percent coupon discounts the pre-tax subtotal
ok 2 - H2 the processor is charged exactly the displayed total
ok 3 - H3 expired coupon leaves the total unchanged and payment still works
ok 4 - H4 disabled coupon is not applied
ok 5 - H5 unknown code leaves the checkout payable at full price
not ok 6 - H6 a user cannot redeem the same coupon on a second order
ok 7 - H7 one user redeeming a coupon does not block another user
ok 8 - H8 applying a coupon to an abandoned checkout does not use it up
ok 9 - H9 a second coupon replaces the first instead of stacking
ok 10 - H10 an amount-off coupon larger than the cart never produces a negative charge
not ok 11 - H11 two open checkouts cannot both spend the same one-per-user coupon
