Fetch sales tax amount for PO lines

Hi All,

We are building Third party application where we fetch Data from Dynamics AX 2012 regarding Purchase Order using AIF web service & Query Service.

We need to fetch sales tax amount for specific po line. The calculation of sales tax involves very complex logic and it is better to use existing code. 

Is there any AIF service exist to fetch sales tax amount for particular poline? If not then what is the other way to get sales tax amount?