We process thousands of credit card payments each month & haven't heard of anyone getting in trouble.
Granted, most of them are in the UK where it's legal to buy cannabis seeds, but we've been shipping internationally for a few months now with no complaints about either credit card problems, customs seizures or anything else.

That could be because either
1. There have been no problems
2. No one's let us know they had a problem
3. Someone had a problem & didn't connect the dots to know it was due to the credit card charge - not like they'd probably tell

Although I hear they don't do anything but destroy the seeds if they're detected in customs, I'd worry about that much more than the credit card charge.
Most company's use a discreet name if you're buying seeds & if a supply store doesn't, it's certainly not illegal to buy hydro supplies.

In fact, with all the hype about going green, being self-sustainable, buying locally & growing your own food, I would think that a few random supplies are going to people who want to grow a few veggies, cooking herbs & such off season.
I can just picture a prepper grow room now.