Was this form of protest uncouth?

I got hit with an extra $100 charge on a $350 subscription I have to have for my business, so total due ended up being $450. I was a couple days past a deadline, as one of my clients paid me very late on a large fee.
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I sent in the $450 but wrote the following, in red ink on the copy of the invoice I sent back with the payment:
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“Math $### – kids gifts for Christmas
.Less.$100 – late fee for subscription
........$### – left for kids gift for Christmas
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Do you really have to charge a $100 late fee?
Think about it!”
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I’m not providing all the numbers but it cut in deep, very deep. I’m three years into a startup company where I provide six jobs to other people. My employees get paid first, company bills next and then me. I have no guilt that finances have been tight, but when hit with excessive late fees I get pissed off.

Was my form of protest uncouth? I understand responsibility about getting things paid, but too many people are getting kicked when their down. Late fees are more often a penalty on those that are struggling than penalty on the irresponsible.