How has this situation turned out?
You can't beat yourself up for sending an ecard with purely good intentions. He should know you well enough to know that you would never mock his symptoms and that if it was interpreted as such, that the problem lies on him, not you. Of course you weren't making light of it. Why would you? Have you ever had a history of doing that before? My assumption is 'no.'
My point being that you're human. Give yourself a break. I do understand, though. As supporters, we expect ourselves to be perfect all the time. We expect ourselves to know what to say and do in any given situation, when in truth, I don't even think our sufferers have any clue sometimes how they'd like for us to respond. In my own experience, my gf never really knew how she wished for me to respond because she didn't even know or understand her own emotional responses many times.
When it comes to PTSD, it's so hard to understand if you aren't a sufferer. And even then, it's still hard for them, too. And like I said, sometimes, they don't always know what's going on with them, either. So how could you ever expect to always know how to say or do the right thing? I think you're beating yourself up way too hard. Don't hold yourself to a level of perfection. Because you will fall short every time.
I hope everything turned out okay in the end. Perhaps he was just busy, and that's why he didn't respond?