why is there a duty tax when sending a box to zimbabwe from los angeles, ca.?

Import taxes and tax treaties.

We charge a duty on most imported goods although it's not all that much for goods coming in from China. There is an import duty on goods coming in from Mexico and Canada too. Although NAFTA is trying to change that.

You probably were sold transportation under the term DDP in which case the shipper is responsible for the destination taxes. If not, you don't have to pay it as I'll go ahead and assume that they released the goods to the consignee without collecting the duty and are looking for someone with money to pay it.