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I could be wrong about this, but I don't actually think that voucher codes are provided when buying games off the PlayStation Store. When going through this channel to purchase games, the only screens that come up are that dealing with the game's description and information, the different buttons/images, and the monetary confirmation/download end result (that, and Facebook stuff if you've enabled it).
When you bought the game, was there a message available that said on the PlayStation Store itself that a voucher code would be provided? If so, you might have to contact Sony about this if they did not send you the code by e-mail, or however the shipment was intended.
