I've some paypal balance in USD. Transfering it to the local bank would not incur a charge but the exchange rate is pretty bad. Therefore, it makes more sense to use it to buy something else.
Today, I tried to buy something in GBP, however, paypal would not allow me to use my paypal balance in USD. After some google work, I realize that if the amount to be paid is more than your balance would allow, it will not let you use the balance but instead, it will charge completely by the credit card!
To solve this problem, I asked the vendor to separate the paypal request into two parts. The first part is less than my paypal balance, so that paypal would allow me to use my existing balance. And the second part is to use credit card.
Quite simple, but of course, since there's a currency switching, I expect poor exchange rate again.