i used to get double time on a sunday in a supermarket
i believe now it depends on your contract if sunday is part of your contracted hours it can be classed as your normal rate however if you are contracted to do 10-2 m-f then do an extra shift on a sunday you should be entitled to their 'overtime' rate which varies from company to company some contracts dont specifiy days and times just x hours over a week so then they are able to manipulate this and not pay extended rates for your hours
contracts in such supermarkets have changed greatly over the past 10 years for exactly this reason its all down to cost cutting im afraid