They will not push you to buy from them, and you aren't likely to get a reply to that question due to that. They support this site to help us, and we are free to go anywhere to get what we need. Other than without a good kit people can't help.
I got my kit from Amazon, THEN found this site. I paid for the Lifetime at that time because I personally felt it was well worth the input I can get so quickly. Since then I have ordered from TFTestkits when I can, but still have used Amazon at times as well if a small order and shipping just puts it out of whack.
In the end, nobody here cares where you got it, as long as you have it, but supporting the site is always nice and they DO support it. I send them my business whenever I can, but they will never ask for it, demand it, and again.. Nobody here will care as long as you have the kit that can give the results they need to help.