Express Checkout don't show vat

In my site www. a c q u a t u a.it when a customer uses paypal express checkout the first page he sees is the login page and on the left there is a resumè of the order. The resumè does not show the vat but only the net price. Some customer gets scared about the fact that the amunts are different and if I am lucky they call me on chat, if I am not, the walk away. Paypal tech says that the software must make a call to the api
This is a google translation of the answer I got:

With regard to your request that I inform you to display the amount of shipments, avt, taxes, fees, cart management etc., the system must receive this information from your cart just as PayPal acts accordingly to the request of your site in the sense that the parameter that is sent in the request will be considered to apply Cart ecommerce amounts, functions and so on checkout.

For additions like the way your Express Checkout, your webmaster will need to implement the API calls and SetExpressCheckout DoExpressCheckout with specific parameters that can be detected in these pages of documentation:

PayPal API Reference
https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/howto_api_reference

specifically in:

SetExpressCheckout (NVP)

and DoExpressCheckoutPayment, the two API calls to PayPal's Shopping Cart

the parameter is PAYMENTREQUEST_n_TAXAMT L_PAYMENTREQUEST_n_TAXAMTm or as appropriate.

If the truck sends the amount parameter value, it is calculated and used in the transaction, note that if the price already include VAT Included, of course not be the case to send the parameter as it would have a double calculation, otherwise, it will be can send the parameters for the amount of total or partial Cart exclusive of VAT which will be added by PayPal.

I have no idea on what should I do to make the modifications.
Can someone help?
Thanks

Go to Admin->Modules->Payment -> PayPal Express Checkout, and change the "Express Checkout Shortcut Button" to off/false.

Are you joking for Christmas?
In this way the express checkout button is not there anymore!!!
It is like to say, do you have a problem with zen cart? then uninstall it!!
Santa i s coming to you but if he sees this answer he will go away!
Merry Christmas!!!!

That is the approach you will need to use if your customers don't understand the process. Zen Cart doesn't presently support the callback they mentioned. That may change in a future version. No ETA at present.

Hi Enzo

I have the same problem with zen cart and cannot get any help from these forums.

If the customer registers then its ok
otherwise, paypal express checkout misses out VAT & shipping.

How did you manage to resolve your problem please.

I am going out of my mind with this.

Thanks

fjhomer (UK)

See my post above: turn off the EC Shortcut button from inside your Express Checkout module settings.