A quick snippet to have the zero prices automatically hidden in the product details page in Ecwid. Add the code into the HTML of your store page, after Ecwid widgets.

ecwid-hide-zero-prices.html

<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.js"></script>
<script>
(function() {
  var __indexOf = [].indexOf || function(item) { for (var i = 0, l = this.length; i < l; i++) { if (i in this && this[i] === item) return i; } return -1; };
  var categories;

  /* if you need to have the zero price generally visible in the store and hide them only in some categories, you can put those categories' IDs, comma separated, into categories array below */
  categories = [];
  /* like
  categories = [2222333, 44455555];
  */

  Ecwid.OnAPILoaded.add(function() {
    var zeroval;
    zeroval = Ecwid.formatCurrency(0);
    Ecwid.OnPageLoaded.add(function(page) {
      var $, _ref;
      $ = window.jQuery;
      if (page.type === 'PRODUCT' && ((__indexOf.call(categories, page.categoryId) >= 0) || categories.length === 0)) {
        $('div.ecwid-productBrowser-price:contains("'+zeroval+'")').css({
          display: 'none'
        });
      }
    });
  });
})();
</script>