There are two kinds of background for this. One is “INT. SUPERMARKET CHECKOUT LINE OL - DAY” and the other is “INT. SUPERMARKET CHECKOUT LINE - DAY”.
The background you are using right now is an OL background, which means the counter overlay that’s already included inside the background will be gone if you include any other overlays of your own. To prevent that, you need to format it this way:
INT. SUPERMARKET CHECKOUT LINE OL - DAY
@overlay SUPERMART create
@overlay SUPERMART opacity 1
@overlay SUPERMART shifts to …
@overlay SUPERMART scales to …
@overlay SUPERMART moves to layer …
If it doesn’t show, chances are your overlay is behind the counter.
If you don’t intend to put any characters behind the counter, you should use this background, INT. SUPERMARKET CHECKOUT LINE - DAY.
If you want to be able to have a character behind the counter and the overlay to be layered on top of the counter, you need to create an overlay for the counter separately.