Calculating Order Grand Total

Order grand total is the final amount the end customer has to pay to the merchant.

Following diagram shows the formula used to calculate order grand total. Please see the pricing fields of an order for exact fields that these values are recorded and the formulas used.

The actual calculations are done in the pricing rules plugin. Implementation projects may change this plugin to have their own logic if required. To read more about implementation details, please read in development section here >>.

Order of calculations

Each of the above items in the order grand total calculation are done in seperate sections in the implementation. Following is the order of the calculations done.

1. Delivery Cost
2. Fees
3. Campaigns (Campaigns may re-calculate delivery cost,fees, order row total and VAT seperately)
4. Order row total
5. VAT, the last step of VAT calculation is to do the Order Grand Total calculation shown above. 

How to initiate order grand total calculation

If you are using web controls provided by Litium Studio to update the shopping cart such as buy button, this calcution is done automatically, and programmer does not have to do anything extra.

However, in checkout-flow template, you need to call a order grand total calculation, when information is available which would change the order grand total. For example, after the delivery address and delivery method is available, one can find the delivery cost.

You may initiate order grand total calculation in two ways.

1. Using order object: call,
   Order::CalculateOrderTotals(SecurityToken token) method
2. Using order carrier: call,
   Module<ModuleECommerce>.Instance.Orders.CalculateOrderTotals(CurrentState.Current.ShoppingCart.OrderCarrier, CurrentState.Current.Token);

When to calculate order grand total:

Order grand total has to be calculated on following occations. All calculations should be done for a new order, which is in Initial state, before confirmation.

1. Adding, removing or changing quantity of an item in the order.
2. When user saves his shopping cart for future use, and reload it when he log in, in a later date.
3. When users personal information changes
4. When the currency of the order changes

When NOT to calculate order grand total

The order grand total calculations should not be initiated after the user has confirmed an order.The exact states a order may follow, depends on your own implementation, in default implementation this is the "Confirmed" order state, which comes after "Init" state.

What ever your implementation will be, there will be a step where the buyer and seller agrees for a final purchase price and related conditions. At this point, a sale is made, and legally binding contract is started. This contract will be in violation, if any of the conditions, prices, anything else changes without the agreement of both parties.

Since, a price re-calculation may involve campaigns, and new prices being fetched from price lists, etc, there is a chance of changing the final grand total if a price calculation is initiated after the order is confirmed. Therefore, it is strongly recomended (though not restricted), NOT to initiate a price recalculation, or change anything related to the order after an order is "confirmed".
Therefore, all information and calculations should be done, before the order confirmation.