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I have a few things i'd like

I have a few things i'd like to ask.
I use Amazon.com.
1) the prices displayed are they automatically changed to the local currency? I coulnd't find anything that could change it back from USD/SGD. It just states $_.__

2) I'm using my POSB GO mastercard. I keep getting a prompt telling me that they couldn't assess the payment method and that I had to update my payment information.

How do I resolve this issue?