Hi,
HikaShop add this parameter to all payment plugins but the plugin as to use this information.
I didn't read the source code of iDeal but :
If the plugin use the "$order->cart->full_total->prices[0]->price_value_with_tax", the price would be good. There's nothing to do.
But if the plugin send every product price, then like the "$order->order_shipping_price", the plugin should send the value of "$order->order_payment_price".
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.