_amount_all is a private function called from a function field. If you need to ihnerith it, you must insert the some function field in your code and in the relative function call the original _amount_all

Thanks for your help, my inherit works perfect now. And a new problem appears, when I try add the shipping collumn ('frais' in french) in updating th price in the order, that works. But, just one time, I can't change the first value submitted.

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()

_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.line': (_get_order, None, 10),
        }, multi="sums",help="The total amount"),
     'frais':fields.float('Frais')
}

purchase_order()

Edit: It works perfect now.

Final code:

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()

_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')
}

The value of amount_total field will only change when there will be any change in purchase.order.line. The field function will not call when there will be any change in purchase.order.So we need to write code explicitly that if there is a change in fraise, the value of amount_total need to be change.So it can be done as follow

'amount_total': fields.function(_amount_all, digits_compute= dp.get_precision('Account'), string='Total',
    store={
        'purchase.order': (lambda self, cr, uid, ids, c={}: ids, ['fraise'], 10),
        'purchase.order.line': (_get_order, None, 10),
    }, multi="sums",help="The total amount"),