I've been buying from Amazon for years. You can use a locally issued credit card to purchase even if it says ITEM IS AVAILABLE ONLY TO CUSTOMERS LOCATED IN THE UNITED STATES, all you need is a valid US address. Here's how:
STEP 1: Use Borderlinx: <click here for link>
. It will generate a valid US address for you to use.
STEP 2: Log in on Amazon. Click on Your Account and Add a Credit or Debit Card.
STEP 3: Enter the details of your card, but don't use your local billing address. Put the US address generated to you by Borderlinx. Click Save.
Congrats, you can now buy from Amazon.
NOTE: Be wary of Region-locked titles. When buying games that can be activated on Steam, check our local Steam store first if the game in question is available/has an accessible Steam page. If not, you cannot activate the game locally.
Ex. I bought Metal Gear Rising: Reveangance on Amazon with zero problems but I cannot activate it on my Steam account because it is not available here in the Philippines.