The problem is that the custom code from lines 81-94 hasn't been updated to mysqli and is still referring to mysql that no longer has a connection available. I think those lines need to be updated.
Try:
<?php
$AddOption = "";
$AddOptionPrice = 0;
if (isset($_POST['ProdOptions'])) {
for ($x=0; $x<sizeof($_POST['ProdOptions']); $x++) {
$ProductOptionsSelected = new WA_MySQLi_RS("OptionGroups",$DBConnection,0);
$ProductOptionsSelected->setQuery("SELECT * FROM optiongroups INNER JOIN options on options.OptionGroupID = optiongroups.OptionGroupID WHERE optiongroups.OptionGroupID = ? AND OptionID = ?");
$ProductOptionsSelected->bindParam("i", "".$_POST['ProdOptions'][$x] ."", "-1");
$ProductOptionsSelected->bindParam("i", "".$_POST['Option_'. $_POST['ProdOptions'][$x]] ."", "-1");
if ($x>0) $AddOption .= "; ";
$AddOption .= $ProductOptionsSelected->getColumnVal("OptionGroupName") .": " .$ProductOptionsSelected->getColumnVal("OptionName");
$AddOptionPrice += $ProductOptionsSelected->getColumnVal("OptionPrice");
}
}
?>