The big supermarkets already open from 11-4 on Sunday's so it's not as big a change as people are making out. Small shops already have regular opening hours on sundays
Speaking as someone who works in retail management and who sees my staff costings budget cut every year as it is, I'd say that from my POV it's a bigger deal than you might expect.
I can't comment on how things are done in other companies but I can say for stone cold certain that if my store were asked to open an extra three hours a week (from the 11-5 we do now to 9-6, for example), the cost of paying a minimum of four staff for those periods of time, plus paying someone to come in an hour before opening to get things switched on and sorted out, plus half an hour afterwards to shut everything down, plus the cleaning staff, plus the increased P&H costs would mean that the standard of service we would be able to provide to our customers the rest of the week would be seriously dented.
Now of course you'd hope, given that the company I work for delivered over £100 million in pre-tax profit last year, that any change would be balanced by a commensurate increase in our store operating costs allowance, but I'd sadly not hold my breath.
On the plus side, it might be the kick up the arse I need to find a better job so really I ought to be all in favour.