Using a flip top lid for your bottle allows you to measure out your liquids without spilling them. I've used a spout or a flip top on my fragrances for years and it has saved quite a bit of waste from spillage!
These will fit the 8 and 16 oz. bottles, but not the smaller 2 or 4 oz. 
 
You'll be able to keep these when your bottles are empty, and use them again. And again. And again. 

These don't seal the full bottles well enough for me to ship them, or I'd replace the current tops. Even with the safety seal, it's just not reliable enough for shipping. BUT, they're perfect to have around the shop for accurate measuring!