I like the Kanger Subtanks a lot. Again, I agree with Supr. But, you now have a different shopping option... you've set a budget of appx $70 + batteries & coils ... You might be better off just getting a single/double battery mod and tank for that, as to me it looks like you're pushing the $100 total mark for this purchase. I wouldn't get fixated on the one vape shop, it does pay at times to look around to see the options. You've got all day because no matter what you decide on it's 99% certain an order won't be processed until tomorrow. Here are some vendors I have used... look at "Starter Kits" and "Mods"That will work too. If it were me, I'd probably scrap the Ijust and buy a Kanger Subtank instead. The tank that comes with the iJust is doubtless better than a Vivi Nova, but not all that great, frankly, and has no coils available except .3 and .5 ohms. That's subohm and deep subohm, and I think you won't like it much. You'll probably make friends with a .5 fairly quickly-- especially on a regulated mod where you can turn it down-- but a .3 you're going to have to work into, and you may never care to try. Having a regulated mod, i don't think you'll get much use out of the iJust battery. Even with your mech background, raw battery voltage is for hobbyists and tinkers; and pretty much useless unless you build coils, or just want a throwaway to avoid damage to better gear. Still worse, with that tank, there is no option to use +ohm coils and your current style. The iStick 100 is a decent piece of gear for what it is and should serve you well. Up to you, buy what you want. Just my opinion.
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What may be out of stock in one store might be in stock in another. It's doubtful you will use the Vivi's once you get new gear. Good luck in your decisions, we've all been there.