Why does the shipping rate sometimes jump when I add one additional small item to the cart?

International shipping carriers often tier their shipping rates by 1lb, and sometimes even less than 1lb.  USPS commonly increases the rate every pound.  If an order has an item that is just under 1lb total weight, adding another item can cause the order's shipping rate to jump to the next tier.  

Unfortunately, the rates set by the post office are ones we have little control over in real time, but we do try to offer promotional discounts to international customers that spend $40 or more on an order subtotal.