What do I do if I cannot make it to a wedding after saying I can?

My classmates at college invited me to her wedding. I got her card a month before the wedding. I sent in the reply card that I can go. Her wedding is this coming Sunday. However, my one of the best friends passed away tragically 2 days ago. I have no energy nor do I want to put up a fake smile at the wedding. I texted her letting her know the news. I was never close to her, but she invited me anyways to her wedding.I asked her for her mailing address so I can send a wedding card with $100. Is this okay? she doesnt have a gift registry. she replied back saying its okay and she understands. is $100 cash okay or in check?