If any of your deliveries were affected by Storm Darragh last weekend, eBay has indicated that they are looking at this, and adjustments may be made.
If that's not the case, and delays are standard across all courier networks, it may be down to high seasonal traffic, but there have been few comments on here about it. I believe one courier (was it Evri? - I'm sure someone will correct me if not) was not accepting further parcels into their network until they had cleared their backlog.
If the delays seem to be particular to one courier, whether you bought the postage on eBay or not, the obvious action to take would be to stop using this courier. Continuing to use a courier when you know it's regularly failing to provide a good service may land you with further late delivery marks, negative feedback, and, even INRs/chargebacks.
So don't just hope eBay will be OK about it as they suggested the courier. If it doesn't work for you, you need to change it.