For the latter, bills have been paid long ago (sunk costs), titles are carried in inventory at or close to zero cost, and direct mail campaigns will create a low cost of goods sold and will be able to sustain a lower rate of response.
A browser is software that is used to access the internet. A browser lets you visit websites and do activities within them like login, view multimedia, link from one site to another, visit one page from another, print, send and receive email, among many other activities.
A browser extension is a plug-in that extends the functionality of a web browser. Some extensions are authored using web technologies such as HTML, JavaScript, and CSS. Others are developed using machine code and application programming interfaces provided by web browsers, such as NPAPI and PPAPI. Browser extensions can change the user interface of the web browser without directly affecting viewable content of a web page; for example, by adding a browser toolbar.