Which item did you purchase?
As a customer, you use "purchase" when you're making a formal complaint to a company:
I purchased one of your two-person backpacking tents through your online store last week, but when it arrived there was a large tear in it.
Another place where you'll often see the word "purchase" is in written descriptions of things that people or companies bought, like in newspapers:
Apple has purchased Quattro Wireless for $275 million dollars.
You can also use the word "purchase" as a noun, without any changes in spelling or pronunciation. For example:
Thank you for your purchase.