You can find defaults and minimum tips setting under the Configuration tab > Settings > Edit Settings, in the control panel.
Go to Edit settings
Delivery Min Tip Percent
this is the minimum tip value in % that the user will have to select
apply only for delivery orders
example if we define 15 here then 15% will become the minimum tip amount that the user will have to pay
The system would automatically set this as the default tip amount for the users to select
The user can change it while placing an order
How this works from a customer's standpoint
When a user opt for DELIVERY service while placing an order,
and further proceeds to pay via PAYPAL or CREDIT CARD, the default is set to 25% (as per to setup value) with the least tip limit set to 15%.
Note: Tip is only applicable for payment via PAYPAL and CREDIT, and not for payment via CASH on delivery mode.
Now, if the user wants to pay lesser than the defined minimum limit using the OTHER option and
Enters a value less than 15% of the entire amount or sets Tip as dollar and enter $1
The error message pops up mentioning the min tip amount and the user can’t proceed without adding equal or above the min value (in the following screenshot, for example, 15% of the total amount makes $17.87, so any value lesser than this will prevent the user to continue the transaction process).