PayPal Express error when trying to pay

Hi,
I'm trying PaypalExpress Checkout plugin but tehere are three problems (note that nothing cahnges wether I do not have a PayPal Business Account: check is set or not):
1)  It displays the "Pay with PayPal" button beneath checkout button. So there are two checkout buttons now. How can I use the Nop standard checkout and remove the PayPal button?
2)  The PayPal Express payment method in payment page is not displayed, although it's enabled
3) When clickong on PayPal button, the payment doesn't procede and reports the errors below.

What's wrong?
Thanks

Short message:   SetExpressCheckoutResponseType returned - Part 1 of 1
The details for the log entry.Full message:   {"Token":null,"Timestamp":"\/Date(1446555455000)\/","TimestampSpecified":true,"Ack":1,"CorrelationID":"725d3934f4d9","Errors":[{"ShortMessage":"Transaction refused because of an invalid argument. See additional error messages for details.","LongMessage":"The totals of the cart item amounts do not match order amounts.","ErrorCode":"10413","SeverityCode":1,"ErrorParameters":null}],"Version":"98.0","Build":"18308778","Any":null}
IP address of the machine that caused the exception.IP address:   77.43.33.125
Name of the customer who caused the exception.Customer:   
Originating page of exception.Page URL:   http://www.glamourcosmetics.it/plugins/paymentpaypalexpresscheckout/submitbutton
The referrer URL.Referrer URL:   /it/cart


Short message:   Error passing cart to PayPal
The details for the log entry.Full message:   10413: The totals of the cart item amounts do not match order amounts.
IP address of the machine that caused the exception.IP address:   77.43.33.125
Name of the customer who caused the exception.Customer:   
Originating page of exception.Page URL:   http://www.glamourcosmetics.it/plugins/paymentpaypalexpresscheckout/submitbutton
The referrer URL.Referrer URL:   /it/cart