How to turn off the invasive third-party “Cashback” alert in the desktop Opera Web browser.
1. First, is it enabled for your version of Opera? Visit eBay or similar, and if you see this invasive element-box adjacent to the Address Bar, then you have it…

2. Top Menu (left hand side) | Settings | Privacy & Security — and then in the Settings search-box search for cashb or cashback
3. You should then be shown the Settings panel section for an unknown service called “Dify”. It appears to be enabled by default. Toggle its button to “off” and then exit Settings.

4. Reload the eBay page and the noxious item should now be gone. The “off” setting should persist for future browser version upgrades.
I assume the process is much the same for a mobile version of Opera.