how do I increase a refund amount to include customers return postage

[bestchoicebrand]

It is not possible, you can return max the same amount buyer has paid in original transaction.

But you can send money twice - first payment back, another return postage.

[reallynicestamps]

The return postage can be refunded by using Paypal's Send Money. Include the eBay transaction number and the PP transaction number for your bookkeeper.