Me and my friend added the same items in our baskets and no matter what payment method we selected got the same unknown error. We then removed the outstation shipment item from our carts, and it worked! Today, I encountered the same error. One shipment was outstationed, gave me the same error. I removed it, it worked. I ordered the outstationed items separately, it worked! Looks like if there is an item which is to be shipped out of station, it gives an unknown error! Reward my QA and fix it!