can you use other countries to purchase in-game items from shop, for example i live in NZ, i switch the shop to USA, can i do that, and will my subway gift card be valid if i switch to USA then purchasing crystals?
No, you will still end up paying the full price as the payment system will automatically charge up for the International price.
For more details as to why this is so you can read here: http://en.tankiforum.com/index.php?showtopic=336209&p=5772076
can i use my gift card from nz to buy from any country that will let me use my subway giftcard, cause in nz they dont have the subway gift card choice :/
Yes, you can.
Customer support service by UserEcho