When your customer adds products to the shopping cart, PrestaShop creates a cart ID, which is sent to Bambora's system as an order ID. When the payment is completed, PrestaShop creates a new order ID – which doesn’t match the order ID in Bambora.
It’s not possible to update the order ID in ePay as it is locked when PrestaShop sends the order information to Bambora.
You can find both cart ID and order ID by clicking on an order in PrestaShop.