If you are going to Germany for shopping, you will have problems paying with credit card. Visa and Mastercards are NOT accepted. Debit or American Express cards are valid. People prefer paying with cash.
Berlin does not have ticket barriers at stations. After purchasing your metro ticket, you need to validate it once before starting your trip on metro lines at the entrance. Ticket inspectors get on train frequently and check the tickets. Even in the middle of night ! “Sorry, we didn’t know” tourist excuse is very popular and not accepted for sure