Wordpress do_shortcode strange behavior - php

Question

I am developing a WP plugin which processes shortcodes and displays amazon item data in place of them. The plugin is working as desired, except for a little strange behavior. You can see my test run at http://passivetest.themebandit.com/test-post-for-zon-plugin/ .
If you scroll down on that page, you can see that 4 "1" s are appended to the content. The shortcode is processed 4 times in this page, and each time WP is adding an undesired 1 to the output. I don't understand why this is happening. There is no "1" anywhere in my html files, and its nowhere in the post content. All my functions just return the content that is to be replaced in place of the shortcode. Can someone please give an explanation for it and let me know how to remove these? Thanks in advance..
My code is as follows:
add_shortcode('zon_product', 'zon_process_shortcode');
// Zon Shortcode processor
function zon_process_shortcode($attributes, $content = null) {
global $zon_html_path;
// default options for shortcode
$options = array('asin' => '', 'style' => 'compact', 'loc' => 'com');
extract(shortcode_atts($options, $attributes));
$asin = $attributes['asin'];
// first find asin in local zon_data table
$zdb = ZonDbHandler::instance();
$product = $zdb->findASIN($asin);
if ($product) {
// if product exists in db, render template
return zon_display_product($product, $attributes);
} else {
// product does not exist in database, get data through amazon api worflow
$product = ZonLibrary::getAmazonProduct($asin, $attributes['loc']);
if ($product) {
// product data has been successfully retrieved and saved, render template
return zon_display_product($product, $attributes);
} else {
// error in fetching product data, check amazon access key id and api secret
$content = include($zon_html_path . 'html' . DIRECTORY_SEPARATOR . 'api_error.php');
return $content;
}
}
}
// Renders selected template with product data
function zon_display_product(ZonProduct $product, $attributes) {
global $zon_html_path;
global $zon_path;
// process other shortcode options
$view_vars = array();
$view_vars['style'] = (isset($attributes['style'])) ? $attributes['style'] : "default";
$view_vars['show_price'] = (isset($attributes['show_price']) && $attributes['show_price'] == 0) ? false : true;
$view_vars['price_updates'] = (isset($attributes['price_updates']) && $attributes['price_updates'] == 0) ? false : true;
$view_vars['hide_unavailable'] = (isset($attributes['hide_unavailable']) && $attributes['hide_unavailable'] == 1) ? true : false;
$view_vars['show_desc'] = (isset($attributes['show_desc']) && $attributes['show_desc'] == 0) ? false : true;
// check if template file exists
if (!is_file($zon_html_path . 'html' . DIRECTORY_SEPARATOR . $view_vars['style'] . '.php')) {
$content = 'ERROR! Zon Template not found. Please check you are using a correct value for the "style" parameter.';
return $content;
} else {
// get product array
$product = $product->getArray();
// if product is unavailable and hide_unavailable is true, return unavailable template
if ($view_vars['hide_unavailable']) {
if ((strpos($product['availability'], "Usually") === false) && strpos($product['availability'], "ships") === false) {
$content = include($zon_html_path . 'html' . DIRECTORY_SEPARATOR . 'unavailable.php');
return $content;
}
}
// render chosen template file
$content = include($zon_html_path . 'html' . DIRECTORY_SEPARATOR . $view_vars['style'] . '.php');
return $content;
}
}