I think you will find with any "worthy cause" you will have resistance, ignorance, and especially with corporations lobbying, influencing, and in the opinion of some controlling our governments this kind of reaction. I would be damn proud of my child being involved in any kind of activism.
Activists from around the globe have endured the most heinous crimes against human rights by governments, corporations, banking cartels etc. especially where profit and control are concerned.
do some research into Fulan Gong practitioners and Chinese activists, whistle blowers who speak out against NWO (media control, fluoride, GMOs, surveillence, etc.) issues for want of a better term.
The smear campaigns, gang stalking, torture, imprisonment etc.
don't allow a couple of automated letters that would be sent to thousands of people deter you from doing what is in your heart.
''Stand up for what you believe, even if it means standing alone"
The exploitation of animals is big business you have to expect this kind of thing.
You should try and look at it positively. or have fun with it.
such as;
Start framing them! view them as little baby steps towards achieving goals.
send them back with; not known at this address

make a scrap book!
use bits and pieces of them in a collage with your favorite quotes and animal/cruelty pics to inspire yourself and others not to lose sight of why you did it in the first place.
Most of all don't worry.. Go and watch some Gary Yourofsky stuff on YouTube and be glad your on the right side.