OK, found the invoice template and edited it.
I attach it below in case someone needs somethign similar.
What I did was basically forcing the invoice to show the price without taxes even if the optino "show prices with taxes" is switched on in config.
for example change
if($display_item_price){
if($this->config->get('price_with_tax')){
echo ' '.$this->currencyHelper->format($product->order_product_price+$product->order_product_tax,$this->order->order_currency_id);
}else{
echo ' '.$this->currencyHelper->format($product->order_product_price,$this->order->order_currency_id);
}
}
to
if($display_item_price){
/* if($this->config->get('price_with_tax')){
echo ' '.$this->currencyHelper->format($product->order_product_price+$product->order_product_tax,$this->order->order_currency_id);
}else{
echo ' '.$this->currencyHelper->format($product->order_product_price,$this->order->order_currency_id);
}
*/
echo ' '.$this->currencyHelper->format($product->order_product_price,$this->order->order_currency_id);
}
I also added a footer for extra text at the bottom of the invoice, to add text there just insert in your language file
INVOICE_FOOTER="some text here"
Please not that I am not a developer and I barely understand how to comment/copy/past php code, so better to check the file before using, but as far as I have tested it works fine.