Adding a button on an OpenERP view is very easy and calling a function on it too. For example I created a function to add cartage amount on the sales order, I created a field to add cartage amount on it and then on button click it will be added to the total amount of the invoice. Lets see how:

Create a function

def get_cartage(self, cr, uid, ids, invoice_id, context=None):
    res = {}
    acc_id = 39
    name = 'Cartage'
    amount = 0.0
    quantity = 1
    disc = 0.00
    val = 'TRUE'
    order = self.pool.get('sale.order')
    line = self.pool.get('account.invoice.line')        
    for invoice in self.browse(cr,uid,ids):            
        if invoice.cflag is False:
            amount = invoice.cartage
            if amount == 0.0:
                raise osv.except_osv(_('No Value !')_('You must add cartage'))
            if amount > 0.0:
                cr.execute('insert into account_invoice_line (invoice_id,name,account_id,origin,price_unit,quantity,discount,partner_id,uos_id,price_subtotal,company_id) values (%s,%s,%s,%s,%s,%s,%s,%s,%s,%s,%s)',(invoice.id,name,acc_id,invoice.origin,amount,quantity,disc,invoice.partner_id.id,quantity,amount,invoice.company_id.id))               
            cr.execute('update account_invoice set cflag = %s where id = %s',(val,invoice.id))
        if invoice.cflag is True and invoice.cartage > 0.0:
            raise osv.except_osv(_('Message'),_('Cartage already added'))
        #raise osv.except_osv(_('No Value !'),_(str(invoice.cflag)))
    return res

Add a button in view

<button name="get_cartage" string="Add Cartage" type="object" />

Note the name attribute, I used name of the function to be called and type as object. This is all you have to do for a function call on a button.