sanef

Does anyone have experience of a sanef toll tag position on the windscreen of a SL 400 or any other Mercedes-Benz with the rear view mirror area completely covered by a plastic cover for rain sensor etc?

Eurotunnel are doing a special offer (Sanef Tolling - Eurotunnel Le Shuttle) on the SANEF toll tag (a little gadget that helps you beat the queues on french toll booths... in theory). Question is, have people found these of use? I usually pay by card at the booths, and can't think of times...

Just been offered one of these via Eurotunnel, at a modest saving, seems like a no-brainer so ordered one. The instructions are very definitive that it should go in the hatched area of windscreen behind rvm. On the CLS there is no spare hatched area to stick the thing, so it will have to go in...