I'd say the plastic bottles are safe. The liquids we use to DIY are not going to degrade the plastic or cause it to "melt". If you are really concerned about plastic, you might want to consider using small glass bottles with dropper tops, but if you live in a house with tile floors like I do, the odds of breakage go up rapidly. I would not worry about those bottles..... they have been in use since the inception of e-
juice. I found a good source for them in bulk is flea bay or there are specialized companies that sell in wholesale lots if you plan on doing a lot of mixing. The cost per bottle goes down big time when buying in bulk. Maybe if you have a local group, you could do an order and split it up among the members.