If the tickets are lost in the post, your seller (the sender) will need to make a compensation claim with the delivery provider and pass the compensation onto you via PayPal. Please contact your seller directly and ask them to do so ASAP. You will find their contact details at the bottom of your purchase confirmation email.
In this case, they are not required to issue you with a refund of the ticket costs.
If you have any issues getting in touch with your seller, please contact our customer support team and we'll take further action.
Click here for information on Royal Mail's claim process.
Click here for information on Australia Post's claim process.