If you are concerned about the privacy of your password and thus your account (which should be the case), you should try to educate the customer service. theThe developer FAQ from the public shaming project for this kind of recklessness lists a few good points and is worth a read.
Also, you should point out that you feel insecure and lose trust in the company and will make them liable for any problems that stem from this no-go.
You should also document that behaviour and try to get a written quote on their point of view if they do not see a reason to fix this. Thus, if any problems arise, it will make the whole thing easier for you from a legal point of view.
Besides that, ifby submitting the sitosite to plaintext offenders, theyyou will probably point out the problem from an externalprovide a third-party point of view, which might help your case.
Also, I assume you use a secure unique password for that site and hopefully have always done so.
If not, treat this as a regular leak, changing all your passwords (and on that occasion, make sure to use unique passwords for each service)