Calculate miles for delivery

Calculate miles for delivery

Hi everybody,
We are in need, after migrating from QuickBooks, to find the way to calculate the miles we drive for deliveries to different customers. Every customer has a fixed distance (no multi-location customers, right now).
We thought about creating a Custom Field (Customers section) with decimal value and total delivery distance (roundtrip).
We don't need to show this value on the invoice.
How is it possible to make this data alive ?
I mean, how can I run a report showing the total miles for deliveries by customer ?

I'm new to this app, but it doesn't look intuitive.
May be there's a very elegant way using the related API, if present on the invoice ? :)
Thanks,
Donato