The Tariffs tab is an editable, dated reference — the withdrawal bands from KES 50 up to 150,000, plus your typical send, Pay Bill and Buy Goods figures. Tariffs change, so the honest design keeps them in yellow cells you confirm against your own M-Pesa app rather than baking a number into a formula that quietly goes stale.
On the Cost Log you pick the type — Withdraw, Send, Pay Bill or Buy Goods — and enter the amount; the charge fills in and a running total builds. Over a month it shows exactly what M-Pesa charges cost you, and the recurring lesson: paying a Till or Pay Bill directly is usually free, while send-then-withdraw pays twice.