Tax on line item

in Development
How do you use the Retail API with Sale to post a tax on a line by line basis? I only see order based, but that doesn't make sense because various items would not have the same tax rate. Is it possible to just set a tax collected amount to add to the sale order?
1 comment
... and by this I mean a cost, not a percentage, third party tax service tells us the "cost" then I have a line by line total taxes collected but it looks like Retail API only has a percentage. Do we have to calculate the percentage and have you do that calculation too?