You can have enough people to handle the average traffic expected that day.
Or you can have enough people to handle the peak traffic moment that day.
Yes, the second one costs you more in labor expenses because you never know when that rush will occur. But look at the pluses.
- You're never under-staffed. You never have to worry about a customer having a bad time and flaming you on Yelp because your staff wasn't able to handle the rush of customers. Whelming? Yes, but never overwhelming.
- You have plenty of extra bodies to do all the other stuff that you never seem to find the time to do. Make a list for your go-getters. Sweeping, dusting, rearranging merchandise, creating fabulous window displays, making signs, tagging merchandise, updating social media, etc.
- You have the ability to exceed customer expectations on a regular basis. To get customers to talk, you have to do more than they expect. Imagine their delight when you have extra bodies to help them shop, wrap their gifts quickly and carry them out to the car.
You can't do all that with average staffing and above average traffic. As for costing you more, if you think of your staff as your greatest asset, the more you invest, the more it pays off. My grandfather had an old adage that served him well for his life - it's impossible to overpay for great help.
Keep that in mind as you do your seasonal hiring.
PS There are ten employees in the above picture (some hidden behind big boxes). There are another ten employees not shown out serving other customers. My payroll is a higher percentage than most stores. I take that money out of my ad budget because delighting customers is every bit as important as a marketing tool as running great ad campaigns.
PPS If you need help hiring a better quality of employees, read the book Hiring and the Potter's Wheel: Turning Your Staff into a Work of Art. It is the method of hiring that has made the biggest difference in the quality of my staff.