There's a problem with NI because Evri charge more, Royal Mail do not. If the seller opts out of using Evri in postage preferences and relists with RM you should be OK. If not ask them to put your ID on their exemption list to remove blocks. They'll find it on their My eBay at Site Map at the very bottom of the page. Scroll down past the categories to Sell > Selling Activities > Manage Bidders