Is it good practice to place a shopping cart along with other primary navigation items in a website? Amazon does this but the USP of amazon is in itself very different from a regular website.
You can see many other e-commerce websites that show the shopping cart in the main navigation. Go check out e-bay, Walmart and other shopping giants.
Since the shopping cart is the gate to the ultimate process of the e-commerce -that is actual purchase- it's quite important to have it presented at all times in a prominent place. The industry's practice is to place is on the top right corner of the screen.