By the hour or day
As you’d expect, with this method you’ll be charged a flat hourly or daily rate for the studio’s services. as an example, a studio could charge Rs5000/- hour or Rs 20000 / day (6 HR) for still images

I strongly advise that you simply don’t enter into an hourly or daily rate arrangement.
To avoid hourly or daily arrangements i like to recommend you spend time refining your image requirements to the purpose where a photographer can understand exactly what images you would like for every product category. this manner they’re going to be ready to quote your project by the merchandise or by the image.