code: public AddOn getAddOn(final String planCode, final String addOnCode) {
          if (addOnCode == null || addOnCode.isEmpty())
              throw new RuntimeException("addOnCode cannot be empty!");
  
          return doGET(Plan.PLANS_RESOURCE +
                       "/" +
                       planCode +
                       AddOn.ADDONS_RESOURCE +
                       "/" +
                       addOnCode, AddOn.class);
      }
docs: Get an AddOn's details
  <p>
  
  @param addOnCode recurly id of {@link AddOn}
  @param planCode  recurly id of {@link Plan}
  @return the {@link AddOn} object as identified by the passed in plan and add-on IDs