Why would you want to use this feature?
You sell car batteries and by selling a car battery the client will get a discount for his/her used battery that they are returning. (used battery needs to be loaded as a trade-in item. Click here for more info on trade in items, ( http://bit.ly/SiStockAdditionalInfo)
When selling the new battery, Smart iT will automatically load used battery.
You sell tyres and when selling tyres, you always need to load wheel alignment.
When selling the tyres, Smart iT will load the wheel alignment automatically.
Steps on how to do the setup:
Go to Invoice > Extra > Invoice rules. (If you do not see it then you need to update your Smart iT Backend.)
Give the rule a name.
Select out either the stock group or stock item. (This is the new item being sold on the invoice)
The rule can affect all the items linked to that stock group or you can select a specific item.
Select the item that should automatically be added to the invoice when selling an item/stock group which was selected in step 4.