for an ISP to tell the difference between what you describe and a single PC running multiple instances of a browser. Frankly, I do not think they care.

To support multiple PCs normally, they have to issue you multiple IPs. That costs them something. That is what they are charging for. Here in Charlotte, it's $10/month.

From the ISP's point of view, with your D-Link, you would be operating over a single IP.