Problems after upgrade of customized site

Hi all,

I was tasked with upgrading a zen cart that was installed by someone else. From version 1.3.7 to 1.3.8a.

After the upgrade, which went smooth, I was missing my faq and testimonials page's, but I found that when I copied the file includes/filenames.php I had written over a few added lines and that was my problem there. So I'm guessing my current troubles are similar.

My problem is this; On the shopping cart page (main_page=shopping_cart) I get the error: (I used the debugging script)

[04-Jun-2008 18:52:14] PHP Warning: include_once(includes/languages/english/modules/order_total/ot_quantity_discount.php) [<a href='function.include-once'>function.include-once</a>]: failed to open stream: No such file or directory in /mypath/includes/templates/XXX/templates/tpl_shopping_cart_default.php on line 32
[04-Jun-2008 18:52:14] PHP Warning: include_once() [<a href='function.include'>function.include</a>]: Failed opening 'includes/languages/english/modules/order_total/ot_quantity_discount.php' for inclusion (include_path='.:/usr/share/php:/usr/share/pear') in /mypath/includes/templates/XXX/templates/tpl_shopping_cart_default.php on line 32
[04-Jun-2008 18:52:14] PHP Fatal error: Call to a member function credit_selection() on a non-object in /mypath/includes/templates/XXX/templates/tpl_shopping_cart_default.php on line 209

Also, in admin > modules > payment, I get the visible-on-the-page error of:

Warning: include(/mypath/includes/languages/english/modules/payment/purchaseorder.php) [function.include]: failed to open stream: No such file or directory in /mypath/admin/modules.php on line 173

Warning: include() [function.include]: Failed opening '/mypath/includes/languages/english/modules/payment/purchaseorder.php' for inclusion (include_path='.:/usr/share/php:/usr/share/pear') in /mypath/admin/modules.php on line 173

I'm guessing I wrote over some customizations when I did the upgrade.

Any help or guidance is greatly appreciated.

It seems to be telling you that you're missing some files, namely:
/includes/languages/english/modules/order_total/ot_quantity_discount.php
and
/includes/languages/english/modules/payment/purchaseorder.php