iDEAL Total amount incorrect error

Hello,

I'm really hoping for a response.
I've been working on iDEAL for weeks now, and still getting the same error: "Total amount incorrect."
I don't see anything wrong with the total amount.
It has to be a TAX related problem because it only occurs when I apply TAX. As long as I have TAX set to 0% I have no problem. The same thing occured with discounts but I've been able to get this to work properly.

This is the error I receive from my bank:

Request parameters:
&itemNumber3=99999999
&paymentType=ideal
&itemPrice2=750
&itemDescription3=Belasting
&itemNumber1=POM00010016V-L
&itemDescription2=Verzenden
&itemQuantity3=1
&urlError=http://www.arte-dal-fiume.com/store/shopping-cart/checkout/?err=1
&itemNumber2=99999998
&itemDescription1=Schaalvierkant16x16cm
&urlSuccess=http://www.arte-dal-fiume.com/store/shopping-cart/confirm-checkout/
&currency=EUR
&itemPrice1=800
&urlCancel=http://www.arte-dal-fiume.com/store/shopping-cart/checkout/?cancel=1
&hash=*I deleted this part*
&subID=0
&itemPrice3=128
&itemQuantity1=1
&validUntil=2012-09-18T8:09:32Z
&language=nl
&itemQuantity2=1
&merchantID=*I deleted this part*
&amount=1550
&purchaseID=*I deleted this part*
&description=Arte dal fiume Store Purchase -

List of errors:
* Totaalbedrag niet juist

The only thing that could be the problem, as far as I can see, is: "&itemPrice3=128"
Because the item price is 800 (TAX included), shipping costs are 750 (no TAX is being calculated over shipping). Makes the total 1550.
128 is the TAX of the itemprice (800).

Any ideas??
I can send you the php file if needed.

Kind regards,
Jolanda