Hi,
For tax calculation , NET price in PO picks by BASB condition type with 362 routine to tax procedure for tax calculation.
Refer how delivery cost post to inventory account and at same time post to different G/L account.
Posting planned delivery cost to Non-inventory account - ERP Operations - SCN Wiki
In you case- you need to play with condition settings:
For your condition type,keep condition category BLANK & Select Accrual check box and
you need to select calculation nature of that condition type where you have field plus/minus in control data-1 segment with values
(BLANK)- positive and negative
X- Negative
A-Positive
As required by you - Just try in sandbox by selecting A-Positive in plus/minus field and check all accounting for further transactions.
NOTE:
If you are happy with all accounting entries, then go for setting in development system.
Regards,
Biju K