Is a clamshell the correct term for a small container of berries?

Pint refers to the amount of volume.

Clamshell refers to a style of container that has a hinged lid, LIKE A CLAM.

You can correctly use either term, no matter what is in such a container, as long as the words you use mean something that is accurate.

Clamshell containers come in different sizes, and are made from different materials. You can buy large or small clamshell containers of berries (strawberries and blueberries for example). Some restaurants offer their to go food in clamshell containers made of card stock or styrofoam.

I call it a small plastic box.

According to Google images, yes it’s a clamshell. Most people here refer to them as a “container of” <whatever>. For most berries, we have a small (pint) and a large size. Both are considered clamshells