Most short fills available have a high VG formula, which is not suitable for many starter kits and pod systems as the e-liquid is too thick. But it is actually quite easy to make any high VG short fill into a 50/50 formula by adding 100% PG nicotine shots.
For a 50 ml short fill, you will need to add two 10 ml bottles of 100% PG nic shots which will create a 6 mg/ml 50/50 overall e-liquid. You may need to decant some of the e-liquid as there is usually only enough room in a 50 ml short fill for one nicotine shot.