alt Hacker NewsSpyware is software that sends information about the user (browsing history, etc) to a 3rd party.
Many affiliate browser extensions do indeed do this, as an extra revenue stream. In fact, I'd recommend never installing a coupon browser extension. But replacing one number with another does not meet the above definition of spyware.
See Spyware: https://en.wikipedia.org/wiki/Malware
"Programs designed to monitor users' web browsing, display unsolicited advertisements, *or redirect affiliate marketing revenues* are called spyware."