It sounds like you haven't been using the ebay Simple Delivery postage labels. To mark the items as dispatched you should open the original SD labels.
Then change the tracking numbers via View Order Details > Postage > Track Package > dropdown arrow > Add Tracking. That has to be done on a desktop browser. Even without doing that the payment will be released after 14 days.
Afterwards if you paid for the original labels you can cancel them here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
but will only get a refund if the postage you used was tracked.
Or if the buyer paid for it it will be automatically refunded to them in 30 days.
Here's more info about SD:
https://www.ebay.co.uk/help/selling/posting-items/setting-postage-options/simple-delivery?id=5575
You may be able to opt out of Simple Delivery during the listing process for some letter or large letter items under £20/100g (excluding the buyer protection fee). I'm not sure if stamps are included in the categories that allow you to do that. You'll have to try it and see.
