coupon bug: after accepting coupon, modifying amount = unwanted situations

[emel]

hi there

please see here: www.wijnwarenhuis.be

made a coupon for 1 product:
http://www.wijnwarenhuis.be/shop/sprankelend/prosecco-extra-dry-detail

the amount of the discount = 8,30 (price of 1 bottle). minimum amount = 49,80 (6 bottles).

now...... ordering 6 bottles gives you 1 for free, using the coupon. BUT if you change the amount AFTER accepting the coupon, the discount is still valid. so if you change it back to 1, you basically don't have to pay(!!!)

hope to receive some good inputs how to tackle this asap.

regards
emel