Gift card adds 2$ to your AppStore account. You may use it only on US accounts.
No expiration date.
There seems to be a difference between the available Store Credit of yours with the site currency you had picked previously. Would you like to:
Itunes & Apple Store GIFT CARD USD $2.99
Itunes & Apple Store GIFT CARD USD $6.99
Itunes & Apple Store GIFT CARD USD $23
Itunes & Apple Store GIFT CARD USD $39
Itunes & Apple Store GIFT CARD USD $54
Itunes & Apple Store GIFT CARD USD $51
Itunes & Apple Store GIFT CARD USD $27
Itunes & Apple Store GIFT CARD USD $38