I found the function who create a invoice in purchase.oder ad I overide it. After, I overide a new time amunt_all function, but for the account.invoice. And, It works!

class purchase_order(osv.osv):
_name='purchase.order'
_inherit='purchase.order'

def _amount_all(self, cr, uid, ids, field_name, arg, context=None):
    res = super(purchase_order, self)._amount_all(cr, uid, ids, field_name, arg, context)
    for order in self.browse(cr, uid, ids, context=context):
        res[order.id]['amount_total'] += order.frais
    return res

def _get_order(self, cr, uid, ids, context=None):
    result = {}
    for line in self.pool.get('purchase.order.line').browse(cr, uid, ids, context=context):
        result[line.order_id.id] = True
    return result.keys()

def action_invoice_create(self, cr, uid, ids, context=None):
    """Generates invoice for given ids of purchase orders and links that invoice ID to purchase order.
    :param ids: list of ids of purchase orders.
    :return: ID of created invoice.
    :rtype: int
    """
    res = False

    journal_obj = self.pool.get('account.journal')
    inv_obj = self.pool.get('account.invoice')
    inv_line_obj = self.pool.get('account.invoice.line')
    fiscal_obj = self.pool.get('account.fiscal.position')
    property_obj = self.pool.get('ir.property')

    for order in self.browse(cr, uid, ids, context=context):
        pay_acc_id = order.partner_id.property_account_payable.id
        journal_ids = journal_obj.search(cr, uid, [('type', '=','purchase'),('company_id', '=', order.company_id.id)], limit=1)
        if not journal_ids:
            raise osv.except_osv(_('Error!'),
                _('Define purchase journal for this company: "%s" (id:%d).') % (order.company_id.name, order.company_id.id))

        # generate invoice line correspond to PO line and link that to created invoice (inv_id) and PO line
        inv_lines = []
        for po_line in order.order_line:
            if po_line.product_id:
                acc_id = po_line.product_id.property_account_expense.id
                if not acc_id:
                    acc_id = po_line.product_id.categ_id.property_account_expense_categ.id
                if not acc_id:
                    raise osv.except_osv(_('Error!'), _('Define expense account for this company: "%s" (id:%d).') % (po_line.product_id.name, po_line.product_id.id,))
            else:
                acc_id = property_obj.get(cr, uid, 'property_account_expense_categ', 'product.category').id
            fpos = order.fiscal_position or False
            acc_id = fiscal_obj.map_account(cr, uid, fpos, acc_id)

            inv_line_data = self._prepare_inv_line(cr, uid, acc_id, po_line, context=context)
            inv_line_id = inv_line_obj.create(cr, uid, inv_line_data, context=context)
            inv_lines.append(inv_line_id)

            po_line.write({'invoiced':True, 'invoice_lines': [(4, inv_line_id)]}, context=context)

        # get invoice data and create invoice
        inv_data = {
            'name': order.partner_ref or order.name,
            'reference': order.partner_ref or order.name,
            'account_id': pay_acc_id,
            'type': 'in_invoice',
            'partner_id': order.partner_id.id,
            # Ajout des frais de port
            'frais': order.frais,
            'currency_id': order.pricelist_id.currency_id.id,
            'journal_id': len(journal_ids) and journal_ids[0] or False,
            'invoice_line': [(6, 0, inv_lines)],
            'origin': order.name,
            'fiscal_position': order.fiscal_position.id or False,
            'payment_term': order.payment_term_id.id or False,
            'company_id': order.company_id.id,
        }
        inv_id = inv_obj.create(cr, uid, inv_data, context=context)

        # compute the invoice
        inv_obj.button_compute(cr, uid, [inv_id], context=context, set_total=True)

        # Link this new invoice to related purchase order
        order.write({'invoice_ids': [(4, inv_id)]}, context=context)
        res = inv_id
    return res

_columns = {
    'amount_untaxed': fields.function(_amount_all, digits_compute= dp.get_precision('Account'), string='Untaxed Amount',
        store={
            'purchase.order.line': (_get_order, None, 10),
        }, multi="sums", help="The amount without tax", track_visibility='always'),
    'amount_tax': fields.function(_amount_all, digits_compute= dp.get_precision('Account'), string='Taxes',
        store={
            'purchase.order.line': (_get_order, None, 10),
        }, multi="sums", help="The tax amount"),
    'amount_total': fields.function(_amount_all, digits_compute= dp.get_precision('Account'), string='Total',
        store={
            'purchase.order': (lambda self, cr, uid, ids, c={}: ids, ['frais'], 10),
            'purchase.order.line': (_get_order, None, 10),
        }, multi="sums",help="The total amount"),
    'frais':fields.float('Frais')
}

purchase_order()

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()

_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, ['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'),
    'frais':fields.float('Frais')
}

account_invoice()