I have earn $600 from Amazon.com associate program and I choose payment method Amazon.com gift certificate/card. My question is can I use it to buy some product in Amazon.co.uk?
No. Amazon.com gift cards are valid only at Amazon.com Amazon.co.uk gift cards are valid only at Amazon.co.uk and so on for all other amazon sites.
... and I would suppose the reason for this has to do with the whole currency conversion thing. Someone, someplace is going to have to pay for to the currency conversion. Either Amazon would have to absorb the cost of the conversion, or they would reduce the overall value of your gift card in order to cover this cost. So, in order to save everyone the hassle, Amazon only ships to locations in their respective geographical location. For this reason, I recommend that people who live in the United States increase the "minimum threshold value" before Amazon sends out a check from CA, DE, FR, or UK. If you receive a $50 check and it costs $15 to covert it to American currency, then it is a pain.
Why do you want to buy on the UK site anyways? I mean if you use the US version of the associates side, wouldn't it make sense to stick with the US side of things? Unless you want to buy stuff that's only available in the UK?
You can still purchase on .com if you're in the UK. I've done it before, it's just a little more because of delivery but with the conversion rates it's not much difference from buying on .co.uk except the delivery time is a bit longer.
you can buy anything from UK if you live in US and vice versa. but the problem is, you can buy it using other payment method except amazon gift card.
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No you cannot use gift cards on any country specific website of Amazon. You can only use the gift cards on amazon.com. Its oneof the terms and conditions of Amazon Gift Cards.