All United credit cards have an annual fee. As you have mentioned cards come with annual fees up to $450. If you fly United often, some perks may cover this high annual fee.
If you need a credit card without an annual fee, you can apply for a regular credit card with travel rewards. Such a card will allow you to earn miles and redeem them with any airline. For example, you may like the Discover it® Miles credit card. The card has no annual fee and using this card you will earn unlimited 1.5 miles on all purchases. All the miles that you’ve earned are matched at the end of the year (provided you are a new cardmember). So if you earn 30,000 miles, you’ll get 60,000 miles. Earned miles can be redeemed for any airline ticket or other travel expenses. This card also come with no foreign transaction fee.