To avoid this problem, you should check out carefully iTunes promo code before that promo code could be expired, some promo codes are tied to a minimum purchase, a first-time purchase, or tied to geography, and other reasons.

It depends! Normally, the percentage that you can save with that iTunes Coupon will be shown right on the page, so you can see if it is applied sitewide or not. However, once you are not sure, try it right on their's products you want to buy.

Usually, Coupon Stacking is not allowed at iTunes. However, the store will surely give out notice if available.

iTunes can offer international shipping for certain purchases but not all orders. Check out their shipping policy to know if your order can be internationally shipped or not.