I'm not sure if the Silk Item is premium or normal. I believe it is premium but I will have it checked for you. (Check here for updated post or admin post later today)

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Silk Item: "Buy Silkroad Online Silk at the lowest prices or earn currency and get it for free. Silk is the exclusive virtual currency to purchase premium items in the Silkroad Online Item Mall. Players may purchase items such as Premium Gold Time Plus, Inventory Expansion, Advanced Defense, and many more!"

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Ultimate Game Card:

If you go into the Billing Methods (Charge) for Silkroad, it should tell you if you can use Ultimate Game Card(s). I believe they allow Playspan (Normal Silk), so it should work there BUT (Note: It appears only for the option of Normal silk for this method).

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It appears Playspan is an exception for premium silk, which suggests you CANNOT use (Ultimate Game Card) for Premium silk. However, you should check if you can use Ultimate Game Card in PayByCash (for Premium silk), you may be able to obtain premium silk, if you can use it there. However, check before you purchase.