Usually, there is a 2 hour refund timer on those items but if you wait too long or take a mod out, the timer will be gone.
If you are a subscriber, you can then send in a ticket and Customer Service may exchange the boots for you. Not every Customer Service agent does it so you have to be lucky, and also keep in mind that it will take a few weeks until they look at your ticket.
To create a ticket, click on the ? in the in-game menu, and make sure to select the "Item" category, not the Bug Reports category. Include as many details as possible, like your character name, your server name, the exact name item you bought and the name of the item you want to have instead. Make sure to place the item in an easy-to-find spot, like in the top of the inventory, and it has to have all of the original mods (armoring, mod, enhancement) inside it or CS cannot refund it.
Good luck!
