In many countries, guests pay the bill with Card, sign the final receipt with desired tips and leave the store. Some processors/gateway allow adding tips on a closed ticket, but many other processors do not.
One way to handle the situation is to Pre-Authorize tickets with some % more amount and capture them. Before capturing transactions server can go to the authorize list and add tips.
How to turn on Pre-Authorization on payment
To enable pre-authorize go to Back Office→ Admin→ Order Type→ edit→ Pre-authorize credit card.
Please note: Once you have captured a ticket you cannot add tips.
The maximum amount of tips
Authorize.net cannot adjust tips more than the pre-authorized amount even though the customer signed it. The administrator can adjust max % tips on the ticket amount in the Back-office->Config->Terminal->Card section.