* Public reaction torn between enthusiasm and scepticism * Beckham shirts already on sale on Champs Elysees * Contract is PSG's latest big-name signing (Recasts with comments from fans, detail) By Vicky Buffery PARIS, Feb 1 (Reuters) - Five minutes before opening at the Paris St Germain store on the Champs Elysees, Alexi Milewicz, 16, is waiting eagerly to grab the first Beckham shirts, already displayed in the shop window less than 24 hours after the star's surprise signing.