Final answer:
The minimum acceptable selling price per unit for the special order, considering avoidable costs and excluding fixed costs that are not affected by the order, is $14. This price would allow Wagner to cover relevant costs and make a profit on the additional units sold.
Step-by-step explanation:
To calculate the minimum acceptable selling price per unit for the special order, we should consider the avoidable costs. Direct materials ($4), direct labor ($5), and the variable portion of manufacturing overhead are relevant in this case. Since two-thirds of the manufacturing overhead is fixed, only one-third of the $6 is variable and relevant, which is $2 per unit. Adding the shipping costs of $3 per unit, the total avoidable cost per unit becomes $4 + $5 + $2 + $3 = $14. This is the cost that Wagner would save if it didn't accept the special order, so any price above $14 would contribute positive margin to cover the fixed costs and add to profits.