The price is not displayed at paypal summary

Hello,

I would like to ask, if anybody know, why is product price display only at the checkout, but if I wanna use paypal gateway, totally amount is zero?
On paypal gateway is field, where customer can fill any amount he want. But, if I fill any price, for example 500 CZK, runs message: “Please enter an amount greater than zero.” and transaction ended.

Please, can someone help me?
You can check here: http://www.studiograzia.cz/products-page/kurzy-tance/

http://wordpress.org/extend/plugins/wp-e-commerce/