I use PhotoCart. (
Photography Shopping Cart | Photocart | Photographer Flash Websites - Pictures Pro)
I am thrilled with their product and customer service. The back end is simple to customize to meet your needs and design elements. My customers simply order from it like a shopping cart (I have this set up to receive credit cards through paypal but you can use other methods). I get a notice in my email to say that I have just received an order, I fulfill the order with my lab, and ship out (the customer pays shipping as an option when they are checking out).
There is a built-in watermarking capability (you can do this totally to your liking - you can watermark the heck out of it or set to reduce opacity on it to whatever you like...)
I don't feel my online sales have suffered. My simple portrait sessions average between $1,000 and 3,000, and I am very happy with that. It beats meeting with customers at my studio which takes away from my family time or my "ME" time, and I am very stingey with that... lol!
I aim to not make any less than $100/hour, so meeting with customers for in-person consults takes away from my profits.