If you edit your eCart object and click "ok" it should update it to the latest version and fix the warnings as long as you have the latest version installed. (make sure to upload the entire WA_eCart folder when you are done).
You probably have different settings in your php7 .htaccess file with a lower memory_limit. If you update it to increase the memory and match your 5.6 version then the error should go away. It is fully compatible with php7.