Just because a store opens on a Sunday doesn't necessarily mean the staff have to work any longer per week than they currently work - you might however require more of them.
Not strictly true. The store needed more staff on a Sunday, however they didn't work only the six hours the store was open. We still did a full shift and got mandatory staff "training" for the other two hours.
Full time staff on early contracts (ie 1995ish) either got time and a half or the option to skip Sundays. Most of them got managed out of the business very quickly when the owners changed. Strange that.