Source: http://www.thatsoftwareguy.com/osc_quantity_discounts.html#CCGV
Q: I am also using the CCGV (Coupon Code/Gift Voucher) Contribution. On the Checkout Payment Page, I get the error:
“Fatal error: Call to undefined method ot_quantity_discount::use_credit_amount() in
……../httpdocs/includes/classes/order_total.php on line nnn”
A: Make the following fix to the file includes/classes/order_total.php.
Change
if ($GLOBALS[$class]->enabled && $GLOBALS[$class]->credit_class)
to
if ($GLOBALS[$class]->enabled && $GLOBALS[$class]->credit_class
&& (method_exists($GLOBALS[$class], ‘use_credit_amount’)))
The location of this line will depend on how many changes you’ve made to the file order_total.php.