I found that GMarket has recently needed the Visa / Master Verified, if you card doesn't registered this already (can register online, you could contact your bank and then could be used immediately), you could not pay.
Shipment fee is only calculated after you confirm you want to place order. So, you just put all the things you want in the shopping cart first, after you finish, then place your order. The shipment fee would be calculated based on the total weight of all the items you ordered.
When the box pop up, sometimes you need to change the screen size to "85%" (at the right bottom corner) since sometimes it is enlarged and you could not see the pop up window in full