GST place-of-supply & tax split
The core mechanic everything else builds on -- get this wrong and every downstream number is wrong too.
Ship-to state, not billing state
Place of supply for goods is the customer's ship-to state -- a gift order, a different delivery address on file, or a centrally-billed corporate purchase never changes this.
Per-fulfillment ship-from state
The seller-side state is resolved from the specific warehouse that actually fulfilled that order, not a single shop-wide constant -- a genuinely multi-warehouse seller gets the right split even when different orders ship from different states on the same day.
Same state -> CGST+SGST, different state -> IGST
Each half of the applicable rate for intra-state, full rate as IGST for inter-state -- computed per fulfillment, then grouped for posting.
Multiple GST rates within one state-day
A state's daily aggregate can span 5%, 12%, 18%, 28% items in the same day -- each rate gets its own Sales + CGST/SGST(or IGST) line-group within the one voucher, never blended into a single number.
Return dates and source tax treatment
Later returns remain dated accounting events. Their tax and location allocation follows the recorded source and configured return workflow.
