I have two Itunes store credits (.99 each) and I would like to use them. How can I redeem these, since this is not an option when I go to purchase a song?

What exactly happens when you try to buy a song ? How items are charged for is automatic, you can't select it : How iTunes Store purchases are billed - Apple Support

When you make a purchase, content credits are used first, as long as the value of the item that you're buying is less than or equal to the value of the credits. Any gift cards are used next. If your gift card is less than the total cost of the purchase, the payment method that you have on file is billed for the remaining balance.

If you are a family organiser in family sharing you can't use an credits/balance on your account