I dont think there is any other method apart from top via paypal.

DO will charge on monthly basis depending on your usage, which is calculated on hourly basis..

You could use Paypal and add a large enough amount of money to your account. You will need to calculate how much that will be based on the package you choose. For example, the smallest package for 1 droplet is $5/month. $5x12 = $60. So if you added $60 to your account (assuming you don't have any other droplets and don't resize) it would basically be equivalent to paying for a year.