This is something I discovered (and love) and it might help some other people too:

You do not have to wait until your chemical runs out to buy more. You don't lose any product. For example, if you have 3 doses of Insta-Grow left and you buy more, you now have 8 doses in your stock.

I like it because you don't have to keep going back to the shop so much.

oh! Thanks! I didn't thought of that!

I discovered this by accident. Not looking properly at what I was doing I bought Insta-grow instead of insect killer. I cursed, as I already had an almost full bottle, and thought I'd wasted my money. I was very relieved when I clicked on it and the tooltip showed the extra doses. I still had to buy the insect poison, though.

i absolutely LOVE this feature of the game

i too discovered it on accident, and now i find that it makes life so so sooooo much easier, esp since i am impatient and am addicted to the insta grow!

I did it, cuz I heard Jr. had like 200 doses of something. Quite useful! Especially with mutation liquid!

That is good to know. I thought it was like Fish Tycoon and you would lose any previous.