Samsung Z3 flip

Took my z3 flip to the mall at Reds MTN store today. The screen randomly changes from one App to the other, now the screen is frozen. I was informed that, for the Samsung technicians to even diagnose the problem, I will need to bring the phone charger (I was willing to purchase at the store) and a copy of the Original Invoice.... I do not have such (the store was able to verify on their records, however insisted that I go, find the invoice) . My trust in Samsung stems from years ago (family and friends), never felt the need to keep invoices that I know can be accessed in-store. Loyalty of over 25years?????.... Is this your, thank you....