During check in the computers to get the boarding pass in Interject, prompted by the computer we reported bringin a musical instrument and the computer indicated we needed to pay for it. Went ahead and try to enter my credit card and it didn't work. We got a receipt saying the transaction could not be prepossessed. We went to drop our bags and we were told we could actually bring the guitar on the cabin freely. Once we boarded the plane was delayed for one hour due to passengers that didn't arrived on time. during the waiting more and more passengers continued arriving all late. Then a woman came looking for my son, who was sitting rows behind me, so I could not hear what happened. Latter he told me the attendant came and threatened him to kick him out the flight and that he would have to fly in another day if he didn't pay in cash for the guitar, even after he explained that the person who did our check in had indicated us to board with the guitar. He paid right there after being embarrassed in front of everybody. Even in the event that we needed to pay for the guitar (and regardless of the fact that their computer did not process our original payment), the attendant attitude was extremely rude and he treated my son publicly as if he had done something so terribly that merited to be kicked out of the plane. I have never experienced such a terrible experience with flight attendants.