I think I found a solution. I added "&& ($row_rspricelookup['price'] != "0")", to the add to cart php code, so the first few lines look like this now:
<?php
// WA eCart AddToCart
if (isset($_POST["segufix_1_ATC"]) && ($row_rspricelookup['price'] != "0") || isset($_POST["segufix_1_ATC_x"])) {
$ATC_itemID = $_POST["segufix_1_ID_Add"];
$ATC_AddIfIn = 0;
$ATC_RedirectAfter = "cart.php";
$ATC_RedirectIfIn = "";
BTW, tried this also with item 35 (only item 13 and item 35 have a $0 line. Item 35 does not get the warning message mentioned in earlier posts, so I am sure the warning message must have been a problem before.
Sorry about the many posts, but this was a work in progress :)