Having a paypal calculating VAT issue? - paypal

Question

I am developing a simple shopping cart with paypal checkout, but i am stuck with the VAT processing part on paypal's side.
Paypal seems to add up total +VAT+ shipping. But when i substract the VAT from the $grandtotal variable in my shopping cart i get the error message:
The totals of the cart item amounts do not match order amounts.
I am using Paypal's SetExpressCheckout.
My questions:
1/ for parameter PAYMENTREQUEST_0_AMT should i use the total with or without the VAT
2/ should i use parameter PAYMENTREQUEST_0_TAXAMT? if so how do i prevent paypal from adding it up
regards

Answer 1

Please see the following extracts from the PayPal site :
PAYMENTREQUEST_n_AMT - (Required) Total cost of the transaction to the
buyer. If shipping cost and tax charges are known, include them in
this value. If not, this value should be the current sub-total of the
order. If the transaction includes one or more one-time purchases,
this field must be equal to the sum of the purchases. Set this field
to 0 if the transaction does not include a one-time purchase such as
when you set up a billing agreement for a recurring payment that is
not immediately charged. When the field is set to 0, purchase-specific
fields are ignored.
PAYMENTREQUEST_n_ITEMAMT - Sum of cost of all items in this order. For digital goods, this field is required. You can specify up to 10
payments, where n is a digit between 0 and 9, inclusive; except for
digital goods, which supports single payments only.
PAYMENTREQUEST_n_TAXAMT - (Optional) Sum of tax for all items in this order. You can specify up to 10 payments, where n is a digit
between 0 and 9, inclusive; except for digital goods, which supports
single payments only.

Answer 2

AMT = item amt + tax amt + shipping amt + handle amt - discount.
So if your item amt already includes tax, then don't use tax amt, otherwise, make sure item amt doesn't include tax and put tax in tax amt.