"Why are you buying that shirt?"
Translation:Dlaczego kupujesz tamtą koszulę?
"kupisz" is a form of the verb "kupić", which is perfective - so it refers to a finished action. It's impossible to use perfective verbs in the Present Tense. What may seem to a learner like it's a Present Tense form is actually Future Simple. "kupisz" means "you will buy".