Its the very reason i will be the last person on this planet, to use online banking.
Everytime you make a purchase, be it for walking gear, or anything purchased online, your internet provider is keeping a watch on what you do.
Nothing online, is really safe from prying eyes, no matter how hard we try to stop popups flashing up on our screens.
Its more of an acute problem if your using a Microsoft PC run computer, as 96% of all computer users and owners use Microsoft software.
Your search engine, be it Google, Yahoo, or Safari for Mac users, are keeping a watch on your every move, and are keeping a background record of it.
Once you have made a purchase, 80% of the time, totally random companies selling similar goods, will send you messages.
You can switch your browser popup off, but it does not make much difference, buy anything off the internet, and you will get companies pestering you with similar products
The best solution, is to just ignore them.