ITEM LEVEL DISCOUNT

Let's take the below example for an order -

ORDER LEVEL DATA

order_subtotal = 200
discount = 100
total_external_discount = 70
Merchant discount = (discount - total_external_discount) = 30
order_total = 178.5

ITEM LEVEL DATA

Item - 1

item level sub-total i.e, total = 120
discount = (item level sub-total/order_subtotal)*(merchant discount)=(120/200)*30=18
Taxable amount = (item level sub-total - discount) = (120-18)=102
Tax = (taxable amount*5% gst) = (102*0.05) = 5.1
Total with tax = (taxable amount + tax) = (102 + 5.1) = 107.1

Item-2

item level subtotal i.e, total = 80
discount = (item level sub-total/order_subtotal)*(merchant discount)=(80/200)*30=12
Taxable amount= (item level sub-total - discount) = (80-12)=68
Tax = (taxable amount*5% gst) = (68*0.05) = 3.4
Total with tax = (taxable amount + tax) = (68 + 3.4) = 71.4

CROSS VERIFICATION

(total with tax of Item1 + total with tax of item2) = (107.1 + 71.4) = 178.5
Which is equal to order_total in the payload.

ORDER LEVEL DISCOUNT

Let's take the below example of an order - 

order_subtotal: 100
discount: 50
total_external_discount: 20.0

Merchant sponsored Discount = (discount - total_external_discount)
i.e, Merchant Sponsored Discount = 30

Taxable amount = (order_subtotal - Merchant Sponsored Discount)
i.e, Taxable amount = 70

Tax on Taxable amount = 70 * 0.05 = 3.5

Merchant Total Collecting from Aggregators = (Taxable Amount + Tax)
i.e, Merchant Total Collecting from Aggregators = 70+3.5 = 73.5

Total you get in the payload = ("Merchant Total" - "Total_external_discount")
i.e, Total you get in the payload = 73.5 - 20 = 53.5

Now on the bill what you have to show is the Merchant Total Collecting from Aggregators. You can simply use the formula,

Receivable Amount From Aggregators = total + total_external_discount

i.e, Receivable Amount From Aggregators = 53.5 + 20 = 73.5