code: public com.google.api.ads.admanager.axis.v201808.BillingSource getBillingSource() {
          return billingSource;
      }
docs: Gets the billingSource value for this Proposal.
  
  @return billingSource   * The billing source of this {@code Proposal}. This attribute
  is optional and
  default value is {@link BillingSource#THIRD_PARTY_VOLUME}.
  If the value is
  {@link BillingSource#CONTRACTED}, the {@link #billingSchedule}
  will be used for
  billing, otherwise the {@link billingCap} will be
  used.
  This attribute can be configured as editable after
  the proposal has been submitted.
  Please check with your network administrator for editable
  fields configuration.
  <span class="constraint Applicable">This attribute
  is applicable when:<ul><li>not using programmatic, using sales management.</li></ul></span>