Physical prepaid visa/mastercard/amex cards bought in the US are by default for domestic use only.
There is currently only 2 prepaid visa that lets you use on international orders, PayPower Travel card and TravelMoney card.
You can find them here:
https://www.paypower.com/StoreLocator/StoreLocator.html
https://usa.visa.com/pay-with-visa/cards/prepaid-cards/travel-money-card.html
As for the "cn gold flashcart", never heard of it. I think you got the name wrong.
If you want to deal with a US vendor, you can use modchipsdirect:
http://www.modchipsdirect.com/
Depends on what your definition of "ripped off" means. Nearly all flashcart vendors use payment processors outside the US, that means you will be charged international fees and exchange rate fees by your card not the vendor.
So ordering something from sites like 3dsgamerworld,
http://www.3dsgamerworld.com/ , say a
R4i Gold 3DS RTS which cost 11.50 USD.
The total would be:
R4i Gold 3DS RTS - 11.50 USD
S/H - 5.50 USD
Foreign Transaction Fee - 2% (0.34, based off of the cart and s/h)(based off of paypower travel card)
exchange rate - variable (this depends on the day an order is placed roughly 2-3 bucks)
Your total you would need on the prepaid card minimum would be 19.34 USD (using 2 bucks as the exchange rate)
Some people would only assume that they would only pay the 17 USD from the cart and s/h. Some consider the 2.34 USD extra charged as some kind of scam charge when it is not.
So that depends on what your definition of "ripped off" means.