At this time, Online Express does not allow for separate billing and mailing addresses. However, you can workaround this on a membership form.
  1. In Online Express, click the memberships tab and click edit next to the desired form. 
  2. On the Extras tab, mark the box to allow gifts of membership
  3. Now, click the fine tuning tab. 
  4. Edit the "Membership is a gift for someone else" to say "Please mail to a different address than billing address" or something similar. (To edit, simply click the field)
  5. Edit the Primary Member field to say "Mailing Address"
Now when someone fills out this membership form, they can indicate a separate address to mail the membership to.

Steps can be viewed here: