Never make it too complicated for the client. Just go with a flat rate or price per hour.
Photography
A place to politely discuss the tools, technique and culture of photography.
This is not a good place to simply share cool photos/videos or promote your own work and projects, but rather a place to discuss photography as an art and post things that would be of interest to other photographers.
Be specific on the usage or they will think that they own the images. Will they be giving images to clients for their own website, social and potential Advertising? I would stick with a per item price and get half the fee upfront to confirm the dates or you'll be fighting over invoices, guaranteed. Do not forget to account for your images processing, editing and delivery or you'll turn resentful quickly. If they are stingy, I'd consider that the first red flag.