The logic your are proposing for such reference line sounds like it could only be a Yes/No (Boolean) result. The expedited shipping cost either exceeded 80% of total shipping or it did not.
I can't think of a reference line showing Yes/No.
But if you just wanted reference line to show the 80% of total shipping cost that can be used as a benchmark for comparing shipping method costs, then it is quite easy - just build your graph, then add a reference line but choose distribution -> percentages instead, and set it to a single 80%.
See image below for an example using Superstore sample data, with the important bits highlighted.