Because Buyers can leave you Negative Feedback every time you cancel a sale, perhaps there is a better way? Most of your items are under £20. Do you know eBay now allows you to set Custom Postage for these? Just enter the correct price, make sure your Simple Delivery is set to 'Large Letter' size, and CP option should become visible. Then you can change it to any size/weight desired.
For the items that are over £20, you can still buy your own CP then add the Tracking Number into eBay. If your Buyer paid for SD label they will be refunded automatically. If you paid, you can request a refund within a fortnight here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller