To Add Extra Fields in account module in openerp use below code
In OpenERP Add Extra fields in account module to create new module name is invoice_cartage and add these 4 file.

1- __init__.py file ---

  import invoice_crtage

2- __openerp__.py file ---

{
"name" : "Account Invoice Inherit",
"version" : "1.1",
"author" : "SACHIN SINGH",
"category" : "Generic Modules/Account",
"website" : "http://www.openerp.com",
"description": "Module to inherit sales order view.",
"depends" : ["account"],
"init_xml" : [],
"update_xml" : ["invoice_order_form_inherit_view.xml"],
"active": False,
"installable": True
}

3- invoice_cartage.py file --

from osv import fields, osv
class invoice_order_test(osv.osv):
    _name='account.invoice'
    _inherit='account.invoice'
    _columns={
             'cartage':fields.float('Cartage',readonly=False),
         'previous_balance':fields.boolean('Show Balance', help="Show User balance on bill"),

            }

4- invoice_order_form_inherit_view.xml.xml file --

<!--?xml version="1.0" encoding="UTF-8"?-->
<openerp>
<data>
    <record id="invoice_form" model="ir.ui.view">
    <field name="name">account.invoice.form.inherit</field>
    <field name="model">account.invoice</field>
    <field name="inherit_id" ref="account.invoice_form">
    <field name="arch" type="xml">
    <data>
          <field name="fiscal_position" position="after">
              <field name="cartage">
              <field name="previous_balance">
         </field>
         </field></field></data>
         </field>
      </field></record>
  </data>
</openerp>