I can understand this from both points of view. I agree it is good to treat yourself now and then but if you had agreed to that next session, and the therapist had dedicated that time to see you and would be out money on the room, then I can also understand her annoyance because for her, she was putting your therapy first but in this instant you weren't.

I don't know how you normally pay for sessions but I don't think it is very respectful to make a therapist wait because you wanted to buy something else , if it was a case of financial emergency then It would maybe be more understandable. I'm sorry if that is hurtful but its how I see it and as someone else said...we don't expect other people to wait so why should our therapist?