Please I need help the item I was purchasing, had a code which will not work when trying to checkout

Hi as above mentioned the item i was purchasing had a code with it which took 10% off  the price and did not expire till the 8/7/18 and it is only the 6th of July. i even took a photo of the screen to show a fellow collector what i was buying. I have committed to buy the item but the Code will not be accepted when going through checkout. Plus why is there GST on shipping, international shipping. Can you please let me know so i can complete sale as i have 100% feedback as a buyer.