I really don't think they (the govt or the bidders) had thought it through properly. Most PayPoint outlets are used to taking money, not paying it out, and don't have secure enough facilities to be able to have tens to hundreds of thousands of pounds on the premises.
Plus, apparently, Mandelson (as "business secretary") made the rather reasonable point that after the current loss of confidence in banks, the PO is in a prime position to (pardon the pun) capitalise on it's reputation for steadiness and the "bombproof" nature of the National Savings products it offers.