How to create Purchase Order of goods with the value of some items highlighted on the invoice?

What is the best practice for creating purchasing documents with the value of some items highlighted on the Invoice, such as: Freight, DIFAL, IPI, etc.

Should these additional values be posted when the invoice is received, via FDC, or should they be included when creating the purchase order?

Problem:

Currently, the purchase requisitions contain the total values that the areas have available for the acquisition of goods.
When it comes to processing a Requisition in an RFQ that will become a PO, all negotiation must be done with values taking into account rule 1.a above.
According to this process, the calculation of the result obtained by the buyer (saving/penalty) considers a gross requisition value (total budget for the acquisition) versus a net award value (without elements that will only be calculated on receipt). In this way, we are not cross-referencing the total procurement budget with the total procurement cost. Comparing different bases generates a distortion in the calculation of the result.