What actually happens when you make an online complaint?

I got a tuna sub yesterday and it was the second time i’ve ordered it. The first time there wasn’t that much tuna so okay, next time I order it, i’ll double it, shouldn’t be a problem. So I did that yesterday on the app. So what is this cause it looks less than a normal amount 😭 i complained about it online, but what happens, if anything? coupon? free sub?