We used a guide in Istanbul recommended by Rick Steves, Metin Koca, and he did not ask for payment by Venmo. We paid in cash (euros), and it was easy. Metin was so easy to work with -- initially via email and later by Whatsapp - and he was a terrific guide. He met us at the port and escorted us on the tram to the Hagia Sophia. We started at 7:30 am at his suggestion, and it was perfect. We were here in June, and there was zero wait and no crowd. We spent about 1.5 hours in the Hagia Sophia, and Metin was very informative and passionate about helping us understand the history. He's friendly, humorous, and went at our pace. He guided us through the Sultanamhet Mosque (Blue Mosque) next -- we only needed 10-15 minutes there- then we toured the fascinating Basilica Cistern. For a rest stop we had tea and some rice pudding at a restaurant near our tram stop, The Pudding Shop, then Metin got us on the tram to return. We chose to hire Metin rather than pay for the excursion our cruise company offered, and I'm so glad we did.