if you're going to use extracts you are going to get nasty withdrawals at some point, hate to warn you but there are plenty of reports on here about extracts and other forums. They are strong (when they are made right) and unlike the plain leaf, you can consume a lot more of the active alkaloids without having the same side effects, creating a much higher dependence than kratom could itself.
look for FST from a reliable vendor, you won't be disappointed but you will most certainly have a withdrawal if you take it constantly for a few weeks or more. kratom extracts are hard to make, tinctures are what you're looking for in general but batches can vary significantly, from very weak to very strong, so make sure to research the tincture from the vendor you are looking at and see how people like it and if the vendor gets reliable tinctures. Tinctures are usually made in bulk and sent out to smaller suppliers who resell them, so they mostly come from a limited amount of sources, so if they make a shit batch, a bunch of vendors may end up getting it as well.
i use kratom once a day usually at 6-8 grams depending on strain. It helps me focus in school and that's about it. I guess my opiate tolerance is just fucked now, however if i don't use kratom i get RLS and diarrhea, so i'm just going to taper it down. My point is that you can get a withdrawal from kratom, particularly if you are using it to come off opiates. For opiate naive users, kratom withdrawal isn't a big deal.
poe - don't buy capsules you are getting seriously ripped off. I sometimes go through 30-40 caps in a day if i'm using kratom the whole day. and it costs me probably $3-4 to do this at most. Buy some 00 or 000 gel caps on ebay or something and buy powdered kratom from a reliable online vendor and you'll save a lot more money.