An offer can tie the purchase of a requested product to that of items that sell slowly. Economic evaluation must include all the combination, with the necessary time and space. The advantage over the main product can be absorbed by ancillary stocks.
It is advisable to reconstruct what quantities are really required and which derive from the conditions of the offer. A prudent prediction of use or sale is required for each component. The possibility of refusing or amending the proposal depends on the specific relationship, to be examined separately.
Let's imagine a store that receives a convenient lot consisting of very different references. Rapid sales of one side can only temporarily fund the waiting of the others. A comparison with separate purchases makes the cost of the combination imposed by the proposal more visible.
I propose to record the result of the whole lot and to review it before the next order. The inventories must maintain the connection with the initial choice. This prevents the success of the main article from making the cost of the products left on the shelves invisible.