Hi,
Thank you for your question.
Yes, you can purchase EasyStore as a separate subscription without buying additional templates. EasyStore licensing is independent from the Templates & Page Builder subscription, so you can add it to your account as needed.
Since your client only requires shopping cart functionality for displaying items (without running a full online store), EasyStore can be used accordingly based on your setup.
If you need help choosing the correct EasyStore plan, feel free to let us know.
Here is the plan
https://www.joomshaper.com/pricing?plan=all
-Regards.