Checking field consistency
Your form contains the vads_amount field. This fields corresponds to the total amount of the order.
In order to make sure your form is valid, the pay gateway checks the specified value.
The applied equation is:
vads_amount = Sum ( vads_product_qty(N) x vads_product_amount(N) ) + vads_shipping_amount + vads_tax_amount + vads_insurance_amount
If the value specified in the vads_amount field is incorrect, your payment form is rejected due to an amount error.