I've used pre-paid cards online for seed several times. I got busted, but in no way was that related to my online purchases. I'll let y'all know if anything about it is in my discovery package tho...
Dude, you want to avoid the "Hydro" shop like the plague. Especially in person. The cops are notorious for staking them out and taking down tag numbers. And it's so much easier to get a court order for the transaction records of a local comapny as opposed to getting that info from a seed bank located in another country. As for the IP thing, use Tor or some other in-private browser.
www.torproject.com It's free, and you'd be a fool not to take advantage of free security, seeing as how you're quasi-paranoid anyway by your own admission. Better safe than sorry.