I created this wizard to add custom invoice in 'pos.order':

import wizard
import pooler
from tools.translate import _

class link_invoice(wizard.interface):

    def _show_report(self, cr, uid, data, context):
        pos_obj = pooler.get_pool(cr.dbname).get('pos.order')
        found = False
        for pos in pos_obj.browse(cr, uid, data['ids'], context = context):
            for inv in pos.invoice_id:
                if inv.state != 'cancel':
                    found = True
                    break
                    continue
        if not found:
            raise wizard.except_wizard(_('Error'), _('POS has no related invoices.'))
        return {
            'report_name': 'invoice.with_picking_lists',
            'type': 'ir.actions.report.xml' }

    states = {
        'init': {
            'actions': [],
            'result': {
                'type': 'action',
                'action': _show_report,
                'state': 'end' } } }

link_invoice('pos.order.invoice.with_picking_lists')

When I try to open invoice, I get this error:

KeyError: "Field '0' does not exist in object 'browse_record(pos.order, 8)'"