Woocommerce- How to insert dummy billing postcode for countries without postcode

Hi,

we have recently set up our online shop selling study materials around the world. Our WooCommerce uses SagePay payment gateway for debit and credit cards. We recently had a customer from Vietnam try to purchase a product numerous times but the transaction would always fail. It turns out SagePay require something to appear in the postcode field even if their location does not have postcodes. In WooCommerce if you select Vietnam as your country the postcode field will automatically be removed, so this option is not even available to fill in.

As a solution I need some guidance on how I can implement a code that adds dummy data for non postcode supported countries so SagePay can take payment.