Yep, you can still use Royal Mail under the new Simple Delivery rules. If you wish to eliminate the other courier, Evri, you can do here:
https://www.ebay.co.uk/ship/prf
This will explain eBay's new Simple Delivery process:
https://www.ebay.co.uk/sellercentre/postage/simple-delivery?mkevt=1&mkpid=2&emsid=0&mkcid=8&bu=43952...
This below, was a recent email message to Private sellers, ( check your eBay messages/ email/ Spam for this) in regarding items of a 2kg weight:
After listening to your feedback about the services that matter most to you, we’re introducing ‘0-2kg small’ parcel size options to provide you with a lower price point. We recommend reviewing and amending your live listings that use small or medium parcels from 20 May, to make sure you’re offering your buyers the best prices. For example, if you have a 1-2kg medium parcel, you can change this to a 0-2kg small parcel if the dimensions fit. You’ll be able to amend this from your Active Listings or View Item page. To get the most competitive prices, consider enabling all couriers by going to postage preferences in your account setting.
@loulouglm