SQL Injection Doesn’t Sanitize But Doesn’t Execute Commands

I am currently doing a pentesting on a web application and focusing more on SQL Injection. This company I am pentesting have a functionality in which we are allowed to buy things from the vendors/suppliers registered there. When a product is added to our cart and ‘Checkout’ button is clicked, the web application will then communicate to the backend to create a cart based on specified ‘cart_id’ and INSERT it to the database. I know this is the case since when I tried to resubmit the request to the server the following error is specified:

"SQLIntegrityConstraintViolationException: Duplicate entry 'RANDOM_ALPHANUMERIC_CART_ID' for key 'idx_cart_id'" 

I tried checking for SQL Injection by adding a single quote at the end of the ‘cart_id’ and HTTP 200 is returned along with server response of a new cart_id with the single quote included. Does this mean It is not sanitizing input? I tried inserting other SQL Commands, the server will still return 200 and the commands are being printed out on the server response but not being executed. Is this web app vulnerable to SQLi (blind?)? If not, Is it possible for me to achieve other vuln such as Stored XSS?

Thank you