Interesting... I buy non prescription items from Entirely Pets.com all the time but I never attempted to buy something like Renal formula food from them since it is readily available within a few miles and I have a prescription. The price on the website is a little less than what I pay but sometimes shipping charges eat up any savings. I might combine some of the food with a few other items next time I need something and get free shipping.
With that said, I would strongly suggest that anyone wanting to change to this diet to consult with their vet and have their cat examined BEFORE making the change even if you don't need a prescription on some websites.