Cash is king in Syria, but travellers' cheques, of course, are safer.
There's no commission for changing cash, but you'll pay per transaction for cheques. You're unlikely to get a cash advance on your credit card, but plastic is increasingly accepted by bigger hotels and stores, and for buying air tickets or renting cars. The tip is the oil that makes the Middle East run. Waiters in better restaurants expect a tip, and if you don't give one they'll probably short change you anyway. People who open doors for you and people who carry your luggage will also expect a tip, but it's up to you to decide if it's worth it.