Threading refers to the screw threading of the battery. Ecigs use different threading, the most popular is 808 and 510, and have the most accessories. Many use a proprietary thread so you won't be able to buy from other vendors.
For a very light smoker like yourself, I see no problem with trying this kit out. At that price it's really not gonna break the bank (my tank costs more than that kit!).
Buying your own liquid and refilling is going to be the most cost effective. For your habit I don't know how necessary it is though.
If you can refill it, there's a few ways. Look up "carto condom refill". The other way is to pop the white cap off and add (drop by drop) liquid into the filler. I'm not sure how large they are, but it probably will hold 20 drops total You'll want to fill it before it runs dry. Add 5-10 drops, let it sit for 15 minutes. Take a look inside, if the filler looks like a slushy then it's done, if not add a couple more (not 10, just a couple), rinse repeat. When it's filled wipe the top clean, blow through it with battery end into a paper towel. This is to clear out the centre tube so it doesn't flood. Thoroughly wipe all connections down. Automatic batteries (no button) can short out easily if liquid gets on the connection, so always clean up after you fill, and it's better to under-fill than to overfill.
When to fill? When the taste starts to change. It's hard to describe, but it will start to taste distinctly different, a little "burnt". If you cook a dry carto you will not be able to get rid of the taste, it's trash.
Cartos don't last forever, even if you take care of them. Yours will last a decent time since it's a once-a-day thing, regular vapers will get a few days-1 week avg. If the taste changes and a refill doesn't solve it, then it's about done.
Welcome to ECF, you found the right place for info, help, and opinions. It can be terribly daunting at first and has a bit of a learning curve, but stick around and no time you'll be in here helping others.
