Two parts in answer is your buyer in Northern Ireland or the Republic - that will make a big difference.

Secondly if its northern Ireland you are better sending with Royal Mail as they wont charge a premium. The price you have quoted at present is an Evri price and there will be an additional premium to pay.

As a seller its your responsibility to set pricing for postage, not us, or eBay so no we cant help with that, either goto your post office or royal Mail online and use their price lists.