VAT problem, template mod help req.

After much invertigation, I have found that cs-cart can't satisfy my VAT scheme requirement by default. So I have started to modify templates.

I am using hook named “orders:items_list_row” in “customer\views\orders\details.tpl”

in this hook, I am calculating products total exc VAT:

{cycle values=",class=\"table-row\"" name="class_cycle" assign="_class"}
{assign var='sum_prices_exc_vat' value=`$sum_prices_exc_vat+$product.base_price`}

My problem I can't return calculated $sum_prices_exc_vat variable back to details.tpl to display its value.

I would appreciate any help