I guess the code in availability_check.v2.js does the check, but because I'm using multiple text fields (to customize a product with user submitted dimensions), it probably can't do the check really. Is there a way to by pass this if I use text field and only execute this if it's with option dropdown or no attribute:

// Ask Drupal if current selection is availible with AJAX post...

var submit_id = "#" + form.id + " .form-submit";

$(submit_id).val(lang_checking).attr({disabled: "disabled"});
$.post(
base_path + '?q=uc_inventory_api/ajax/can_add',
eval("({"+params.join(",")+"})"),
function(data){ toggle_submit(data, submit_id); }
);