Sometimes extension conflicts can be caused by via an observer conflict on the add to cart observer. You can try this:
File: app/code/community/ITwebexperts/Payperrentals/Model/Observer.php
/*This part can be needed when the observer is executed twice because of some extension*/
//if (Mage::registry(‘update_executed’)) return;
//Mage::register(‘update_executed’, true);
uncomment (delete the // ) on these 2 lines so it reads:
/*This part can be needed when the observer is executed twice because of some extension*/
if (Mage::registry(‘update_executed’)) return;
Mage::register(‘update_executed’, true);
