Flavourless nic shot. 18 mg/ml nicotine in a 10 ml bottle. 70/30 VG/PG. Simply mix nic shots with your zero strength flavoured short fill e liquid to create larger amounts of e liquid in the flavour and strength you choose.
For example, mix your 10ml nicotine shot in an 18 mg/ml strength with 50 ml of zero strength e liquid in your chosen flavour to create 60 ml of e liquid in a 3 mg/ml strength.