Best program for basic till menu

Hi there & apologies if this post is in the wrong place.

I wish to create a basic till app to add up menu food items and display a total.

This only needs to be buttons for each menu item (25) that can calculate the total so we can give the correct change.

The software we currently use is called iZettle that runs on our iPad but we could cope with something much more basic for when we lose our wifi connection.

Many thanks in advance

Ben