def do_delivery(self, cr, uid, ids, context=None):

        self.write(cr, uid, ids, {'state':'on_delivery'}) #change state to On Delivery in Odoo

        return True

<?xml version="1.0" encoding="utf-8"?>

<openerp>

    <data>

        <record id="act_delivery" model="workflow.activity">

            <field name="wkf_id" ref="sale.wkf_sale"/>

            <field name="name">OnDelivery</field>

            <field name="kind">function</field>

            <field name="action">do_delivery()</field>

        </record>

        <record id="trans_ship_delivery" model="workflow.transition">

            <field name="act_from" ref="sale.act_ship"/>

            <field name="act_to" ref="act_delivery"/>

            <field name="signal">do_delivery_button</field>

        </record>

        <record id="trans_ship_delivered" model="workflow.transition">

            <field name="act_from" ref="act_delivery"/>

            <field name="act_to" ref="sale.act_ship_end"/>

        </record>

    </data>

</openerp>