(NY) Employer just mailed me a stale check from February

(If it's relevant: for the purposes of the part-time work I'm doing, I'm classified as a contractor rather than an employee.)

I work part time during the fall, winter and spring as event staff for an institution of higher education near me. The institution is sometimes a little slow paying me, but they do eventually pay and I'm okay with that - except for one instance, where it would appear the institution's accounts payable department lost track of one of the checks they were supposed to send me, because it's dated mid-February.

I just got it last week. The envelope is postmarked mid-September, so it's not a postal service snafu. It looks like somebody in the office said "oopsie" and just popped the check in the mail.

Fat lot of good the check does me, because my credit union is refusing to process the check as it's over 180 days old.

I've contacted the institution's accounts payable department and haven't heard back yet (it's been a week), so is there a way to light a little fire under them so that I can get my money?