Go to your
product.tpl and look for the code that reads
Code:
{if $product.price ne 0}
<font class=ProductDetailsTitle>{include file="currency.tpl" value=$product.price}</font><font class=MarketPrice> {include file="customer/main/alter_currency_value.tpl" alter_currency_value=$product.price}</font>{if $config.Taxes.use_vat eq "Y" and $product.vat gt "0"}, {$lng.lbl_including_vat} {$product.vat}%{/if}
{else}
<input type=text size=7 name=price>
{/if}
Comment out the else part (like this)
Code:
{if $product.price ne 0}
<font class=ProductDetailsTitle>{include file="currency.tpl" value=$product.price}</font><font class=MarketPrice> {include file="customer/main/alter_currency_value.tpl" alter_currency_value=$product.price}</font>{if $config.Taxes.use_vat eq "Y" and $product.vat gt "0"}, {$lng.lbl_including_vat} {$product.vat}%{/if}
{* Remove Input Box
{else}
<input type=text size=7 name=price>
*}
{/if}
Then just set the price of that item to $0.00