A seller doesn't know which shipping option the buyer will select at the time of making AddOrder call, how can the "Total" be calculated?
Summary
The Total property is an estimate cost amount for the Order you are creating. The actual default Seller Total that shows on the buyer's Review Your Purchases from ${SellerID} >Review payment details page for the order created by the seller is not determined by the Total value you defined, the Seller Total is calculated by the transaction subtotal plus all the costs you specified in your ShippingDetails container (AddOrder ).
You should set the Total value to be greater or equal to the TransactionPrice total of all the transactions you attempt to combine in your AddOrder request. Otherwise you get a request Error 12104 as below.