[Swoon]

It says 5.5g
It was around 80 €

Is the item transparent ?  I mean if it's a plastic case or a cartoon box?
If it is transparent , u will probably succeed.
There are another members here claiming EB works with Western Digital, try to see in the website their terms about refunds, go to the wayback machine and try to see if they have changed that.
If they didn't change anything, there is a high chance that EB is still succeeding to date.

What specifically does the store's return policy have to say to know if EB works there?)

"The company assumes responsibility for delivering the goods to your specified location (DDP terms).
In this case, an empty box would likely be considered a delivery failure, and you'd be eligible for a full refund."
Search for something like that, they will probably have to assume that they didn't ship the correct item, try to see if they pass the responsibility to the carrier.
If they claim that they ship the item and carrier has POD, then u will probably ask proof or any backup that backup of their claims, photos or something, because u didn't receive ur product.
If all this doesn't work, proceed with a bank dispute and provide evidence.