An avoidable cost is the type of cost that you do not need to pay for if in case the activity for it does not occur. For example, there is one activity that did not really happen; this means that you do not have to pay for it anymore.
For example, the expenses that you would need to pay for supply can be considered to be avoidable, especially if there are some supplies that you do not need to get. When you say unavoidable costs, this is the type of cost that you do still need to pay for, even if the activity has not taken place.
For example, there may be a product that is not performing well, and it will be dropped, but because supplies and the production for the product have already taken place, the costs for the product can be considered as unavoidable.