fixed an marginal cost calculation issue
The marginal cost is 3. Marginal costs do not depend on the fixed cost, and when your variable costs are constant, then the marginal cost and the variable cost are the same. Note that your total cost is $C=FC+3q$ and the marginal cost is always the derivative of your total cost, in this case, $3$.

As for the fixed costs, 4000 is definitely part of it, but maybe also the forgone profits from selling the goods in the various stores at sub-optimal prices, as pointed out by another response. Other's have pointed out that if you calculate the variable profits for each price, 9 gives the largest profits, so in all the other stores that you made less profits, the lost profits should probably be added to the fixed cost.