There is a form to complete every time you don't use Simple Delivery and you pay the postage.
Are you completing the form with all the details, including adding the tracking for the service you did use?
Details and a link to the form that needs to be completed below. There shouldn't be any need to use the automated assistant and you can't void the label.
If you selected ‘Seller pays’ and offered free postage on your listing, you'll need to fill out the Request a refund for Simple Delivery form for the label that wasn’t used. Make sure to send your request within 14 days of your item selling
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)