Hi! Totally showing up out of the blue after anonymously following this forum. But I think this is always a tough choice, because invites flurry in so randomly. I think it speaks to the program/PI that they are being inflexible. I think you have to weigh what you value in a program versus the other program you have already committed yourself to going to interview at. From my mentors, I have heard that skype interviewing in place of an in-person interview can be interpreted as lack of interest, lack of commitment, and they feel like they don't get a whole sense for your person. In the same respect, some people quite literally don't have the means to pay to attend all of their in-person interviews. If financial means aren't an issue for you, maybe push on coming an alternate weekend. My experiences so far with various programs has been wonderful in understanding that you may have received other interviews from other schools and that's okay.