Problems With Payment Using Download Card

I have used a download card which left a balance on my account. I tried to purchase an item from the app store whereby the price was greater than the balance on my account. I have a paypal account and had used the paypal previously to purchase apps. However on this occassion, I could not pay the balance with a card or paypal account. An error message kept appearing stating that there was not enough money available in my leapfrog account but did not give me an option to pay the balance. It would appear that the only option was to purchase another download card. I do not wish to take this option and prefer to pay the balance either with paypal or visa card. Can you advise? Many thanks Majella