code: public com.google.api.ads.admanager.axis.v201808.BillingBase getBillingBase() {
          return billingBase;
      }
docs: Gets the billingBase value for this Proposal.
  
  @return billingBase   * The billing base of this {@code Proposal}. For example, for
  a flat fee
  {@link BillingSource#CONTRACTED contracted} {@link
  #billingSource}, set this to
  {@link BillingBase#REVENUE}. This attribute is optional
  and defaults to
  {@link BillingBase#VOLUME}.
  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>