A proxy-switching browser extension keeps profiles: one direct and one through the purchased address. Instead of editing system settings, you switch profiles.
How to set it up
In the new profile, pick HTTP, fill the address and port 20001, and enter the username and password. For SOCKS5, use 20481. The browser only leaves through the address while the profile is active.
The extension is third-party software and you install it; we deliver the line that goes in the profile.