Можешь вот так сделать (это для radio)
<script type="text/javascript">
$(function(){
$(jQuery.unique(
$('input:radio')
.map(function(i,e){
return $(e).attr('name') }
).get()
)).each(function(i,e){
$('input:radio[name="'+e+'"]:visible:first')
.attr('checked','checked');
});
});
</script>
А если php, то там меняешь 189-199 строка
<?php foreach ($option['product_option_value'] as $option_value) { ?>
<div class="radio">
<label>
<input type="radio" name="option[<?php echo $option['product_option_id']; ?>]" value="<?php echo $option_value['product_option_value_id']; ?>" />
<?php echo $option_value['name']; ?>
<?php if ($option_value['price']) { ?>
(<?php echo $option_value['price_prefix']; ?><?php echo $option_value['price']; ?>)
<?php } ?>
</label>
</div>
<?php } ?>
на это
<?php
$i = 0;
$len = count($option['product_option_value']);
foreach ($option['product_option_value'] as $option_value) {
if ($i == 0) {?>
<div class="radio">
<label>
<input type="radio" name="option[<?php echo $option['product_option_id']; ?>]" value="<?php echo $option_value['product_option_value_id']; ?>" checked/>
<?php echo $option_value['name']; ?>
<?php if ($option_value['price']) { ?>
(<?php echo $option_value['price_prefix']; ?><?php echo $option_value['price']; ?>)
<?php } ?>
</label>
</div>
<?php } else { ?>
<div class="radio">
<label>
<input type="radio" name="option[<?php echo $option['product_option_id']; ?>]" value="<?php echo $option_value['product_option_value_id']; ?>" />
<?php echo $option_value['name']; ?>
<?php if ($option_value['price']) { ?>
(<?php echo $option_value['price_prefix']; ?><?php echo $option_value['price']; ?>)
<?php } ?>
</label>
</div>
<?php }
$i++;
}?>