COVID-19 can do more than torment patients physically. It also clobbers some financially. Even though many insurers and the U.S. government have offered to pick up or waive costs tied to the virus, holes remain for big bills to slip through and surprise patients. People who weren’t able to get a test showing they had the virus and those who receive care outside their insurance network are particularly vulnerable. Who provides the coverage and how hard a patient fights to lower a bill also can matter. Experts say most patients will incur few costs because they have mild cases.