I joined this forum SPECIFICALLY to answer this question.
I use
https://noscript.net for Firefox to block ads and unwanted javascript stuff. You'll need to find your own equivalent for other browsers.
The problem is that alicdn.com loads masses of page content including this popup. What it actually delivers, if you inpsect the code, is an iframe with a mask, so you have to click on the coupon, and the coupon itself. Fortunately noscript allows us to target what gets blocked.
From the Aliexpress.com page open the noscript settings.
For the alicdn.com choose a custom setting.
Turn OFF the frame setting.
That
should remove the coupon popup for good!