PhD interview - travel arrangement
Take the coach, stay overnight and pay your own expenses; forget this flap ever happened; do your best in the interview.  Keep in mind that the office staff who arranges for travel is probably overworked, and has NOTHING to do with hiring decisions, and will have almost nothing to do with you once you are working there.  Focus on figuring out if the job is a good fit for you -- read up about the department or whatever before you go, think about how you can make a contribution to their efforts, and how you will convince them of that.  And enjoy your trip.  Even if this is not the place you end up working, the interview will be valuable experience for you in your job search.