Many restaurants add a service fee to offset the cost of doing business. With this, some have no-tipping policies, meaning they cover expenses and an 18-20% tip in the price.
In these cases, it is not obligatory to tip, but tipping an additional percentage for outstanding service is always appreciated.
Ideally, the restaurant will specify where the service fee is going. However, a service fee on a takeout order is not a tip.
Someone had to take your order by phone or through an online system, and an employee cooked and packaged your food. This is a service, and in general, a 15% tip is appropriate.
Always ask for clarification rather than assume that a tip is included. Enjoying a restaurant is a privilege, and tipping well, even when there is a service fee, is crucial.