Why do the prices on Japanese Pokemon boxes seem to change all the time? 

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There are two reasons for that. The first is that the Japanese Yen to US Dollar exchange rate constantly shifts. Second, the demand for Japanese booster boxes in both Japan and the USA will also shift the price of the boxes.

 

Also, keep in mind that the Pokemon Company in Japan makes different decisions than the US Pokemon Company, so all these factors impact the prices. It is safe to assume that the price you see now, WILL go up or down the next week, month, or even day depending on how crazy the market is. 

DUE TO THE VOLATILITY OF JAPANESE PRODUCTS, PRICE ADJUSTMENTS WILL NOT BE HONORED. EVEN IF THE PRICES CHANGES OVER NIGHT.