Why are there 5 options to buy voting tickets in the shop, starting with 1K per 10, ending with 3K. Are they different? Or is it just so you have to pay more if you hit the exchange limit?

Also, what's up with the captcha on this forum, it expires in like 1 minute while I'm writing the post, and wants me to do the image crap every time (yes I know I can check it when I'm about to submit, but still... why?)