Use the following jsonpath query:

    serviceLines[0].customerProducts.findAll{product -> product.highLevelProduct == 'HSI'}.customerProductIdentifier


Test:

    System.out.println(js.get("serviceLines[0].customerProducts.findAll{product -> product.highLevelProduct == 'HSI'}.customerProductIdentifier").toString());

Note, that it will return list of identifiers in case your JSON contains several objects meeting your requirement.