Field for additional charge the drupal / drupal commerce way What is the canonical drupal commerce way of storing data for say something like an additional charge? 

Would it be a price field / text field / Line item field? 

I would like the field to store data like this:

label: color charges
desc: field stores the additional amount for a color foobar product
number of values: 1

example value: 0.75
You don't make the use case very clear, but sounds like you could use some of the existing price fields, fx unit price on product line items, but simple add the price with a custom component. That way the price would be viewable in the order overview, but when seeing the product cost you get the simple and correct value of $ x.xx.