I can't help with the RTS cart problem sorry, but have you looked at Box Elder Chemical? They have got a 50% off sale on until Sunday. I bought a whole litre on their last sale and it is good stuff, and I paid around one third of what I would have in the UK.
I have been DIY'ing for a while so can help a bit more there. 'Exact' is a good thing with DIY otherwise you are never going to be able to reproduce your recipes reliably. My wife and I both vape around 20ml a week and this is the batch size I make up. I use 30ml bottles, and 5ml, 2ml and 1ml syringes and gloves. Goggles are recommended too if you are using 100ml nic. Definately get the 1ml syringes as they are the most accurate for the small amounts of flavouring required in small batches. Pipettes? hardly ever use them, same for measuring cylinders. A good selection of syringes and needle sizes are more than enough unless you are going to mix large amounts at once. Small plastic funnel is useful.
You'll need unflavoured nic at your chosen concentration, flavourings (best use proper vaping flavours from either Flavourarts or Perfumers Apprentice), plain PG and plain VG.
Write down every recipe you try as you make it. Before making up a juice double and treble check your calculations to make sure you are not going to poison yourself with too much nic. I use 48mg nic which is strong enough in my opinion. If you use 100ml be doubly careful!
If you are not too hot with the math use a good ejuice calculator as a check. Google up eJuicemeup, that's the one I use for the more complicated recipes. Double check each ingredient quantity as you add it to the mix. Shake the final mix very well, let is stand for a few minutes and shake again before trying it. If it doesn't taste perfect leave it to stand for a few days and try it again, you will be surprised how a flavour will mature. Never add too high a percentage of flavours to start with. Start low and taste, you can always add more but they are a ..... to get out again
Always make up small amounts to try flavours. I make up one or two mil mixes of just PG, VG and flavours to test with, don't waste your expensive nic concentrate until you find a flavour you want to take further. Use a normal atty and drip on it for testing the flavours. What smells wonderful in the flavouring bottle may taste awful and vice-versa so always do the drip test.
Only when I have a flavour I like do I make up a larger amount of 5ml with nic liquid included then vape that for a day or so. If I still like it after that only then will I make up a normal 20ml size mix.
I can't think of anything else vital at the moment but I'm sure someone else will chip in if I've missed anything. Good luck, and don't be disappointed if your first mixes taste rubbish. Like all of vaping there is a learning curve but when you master it it really is fun and satisfying to vape your own personally tailored flavours. And it is hugely cheaper - in the UK a 10ml bottle of premixed eliquid costs around £4 to £5 - I make the same amount for around 60p
