Yeah, it worked for me before. But sometimes I can't buy from the online store ; if that happens, I use the app and it seems to work. Once in a while, it doesn't work at all. I don't remember the last time it failed to make a purchase. And if I use a credit card to make up the difference in the case that my Steam wallet isn't enough, the purchase will always go through.
It's odd that you can purchase from the US store at all as the payment method needs to match the region (this is also true of Steam Wallet -- e.g. I can't purchase from Steam US despite Steam AU also using USD).