I found, in fact I was to use the amount_residual function and add the shipping price on the calcul. That give"s the following result.

class account_invoice(osv.osv):
    _name='account.invoice'
    _inherit='account.invoice'
    def _amount_all(self, cr, uid, ids, name, args, context=None):
        res = super(account_invoice, self)._amount_all(cr, uid, ids, name, args, context)
        for order in self.browse(cr, uid, ids, context=context):
            res[order.id]['amount_total'] += order.frais
        return res

    def _get_invoice_line(self, cr, uid, ids, context=None):
        result = {}
        for line in self.pool.get('account.invoice.line').browse(cr, uid, ids, context=context):
            result[line.invoice_id.id] = True
        return result.keys()

    def _get_invoice_tax(self, cr, uid, ids, context=None):
        result = {}
        for tax in self.pool.get('account.invoice.tax').browse(cr, uid, ids, context=context):
            result[tax.invoice_id.id] = True
        return result.keys()

    def _amount_residual(self, cr, uid, ids, name, args, context=None):
        res = super(account_invoice, self)._amount_residual(cr, uid, ids, name, args, context)
        for invoice in self.browse(cr, uid, ids, context=context):
            if invoice.move_id:
                res[invoice.id] += invoice.frais
        return res

    def _get_invoice_from_line(self, cr, uid, ids, context=None):
        move = {}
        for line in self.pool.get('account.move.line').browse(cr, uid, ids, context=context):
            if line.reconcile_partial_id:
                for line2 in line.reconcile_partial_id.line_partial_ids:
                    move[line2.move_id.id] = True
            if line.reconcile_id:
                for line2 in line.reconcile_id.line_id:
                    move[line2.move_id.id] = True
        invoice_ids = []
        if move:
            invoice_ids = self.pool.get('account.invoice').search(cr, uid, [('move_id','in',move.keys())], context=context)
        return invoice_ids

    def _get_invoice_from_reconcile(self, cr, uid, ids, context=None):
        move = {}
        for r in self.pool.get('account.move.reconcile').browse(cr, uid, ids, context=context):
            for line in r.line_partial_ids:
                move[line.move_id.id] = True
            for line in r.line_id:
                move[line.move_id.id] = True

        invoice_ids = []
        if move:
            invoice_ids = self.pool.get('account.invoice').search(cr, uid, [('move_id','in',move.keys())], context=context)
        return invoice_ids

    _columns = {
        'amount_untaxed': fields.function(_amount_all, digits_compute=dp.get_precision('Account'), string='Subtotal', track_visibility='always',
            store={
                'account.invoice': (lambda self, cr, uid, ids, c={}: ids, ['invoice_line'], 20),
                'account.invoice.tax': (_get_invoice_tax, None, 20),
                'account.invoice.line': (_get_invoice_line, ['price_unit','invoice_line_tax_id','quantity','discount','invoice_id'], 20),
            },
            multi='all'),
        'amount_tax': fields.function(_amount_all, digits_compute=dp.get_precision('Account'), string='Tax',
            store={
                'account.invoice': (lambda self, cr, uid, ids, c={}: ids, ['invoice_line'], 20),
                'account.invoice.tax': (_get_invoice_tax, None, 20),
                'account.invoice.line': (_get_invoice_line, ['price_unit','invoice_line_tax_id','quantity','discount','invoice_id'], 20),
            },
            multi='all'),
        'amount_total': fields.function(_amount_all, digits_compute=dp.get_precision('Account'), string='Total',
            store={
                'account.invoice': (lambda self, cr, uid, ids, c={}: ids, ['frais'], 10),
                'account.invoice': (lambda self, cr, uid, ids, c={}: ids, ['invoice_line'], 20),
                'account.invoice.tax': (_get_invoice_tax, None, 20),
                'account.invoice.line': (_get_invoice_line, ['price_unit','invoice_line_tax_id','quantity','discount','invoice_id'], 20),
            },
            multi='all'),
        'residual': fields.function(_amount_residual, digits_compute=dp.get_precision('Account'), string='Balance',
            store={
                'account.invoice': (lambda self, cr, uid, ids, c={}: ids, ['invoice_line','move_id'], 50),
                'account.invoice.tax': (_get_invoice_tax, None, 50),
                'account.invoice.line': (_get_invoice_line, ['price_unit','invoice_line_tax_id','quantity','discount','invoice_id'], 50),
                'account.move.line': (_get_invoice_from_line, None, 50),
                'account.move.reconcile': (_get_invoice_from_reconcile, None, 50),
            },
            help="Remaining amount due."),
        'frais':fields.float('Frais', readonly=True)
    }

account_invoice()