Enable you to book a Southern Ireland address on a Northern Ireland service - first you know of it you get a bill for X2 the original cost (plus you have paid that already).
Website should be robust enough to find errors like this and not let you continue. My tiny parcel cost a fortune to send as a result and company just told me to pay or else. Other websites I use would flag it up as an incorrect address, and would not accept the booking. This one enables this to happen and then you incur additional costs.
So be careful that your bookings are very accurate.