Many stores will not allow you to buy gift cards in the store with a credit card but you can buy them on the store's website. I've done that at Staples and shipping is usually free if you spend enough.

You should be able to pay whatever amount you want on the Comcast and AT&T websites. I pay my Verizon bill with AMEX but Charter doesn't accept AMEX. You can use the gift cards you buy to pay vendors who don't accept AMEX.