Extra flavor shots also generally make me not want to order. Someone who has perfected a recipe isn't going to let you mess it up.
I am not defending any particular company, but the extra flavor shots aren't always meant to simply add more flavor. If you ever try DIY, you will understand that typically adding more flavor, actually lessens the flavor that you taste, or simply makes it worse. The extra shots are intended more for people that want to blend their juice after they receive it. ie: someone orders a 15ml bottle of grape with double the flavor, in 18mg strength, they add 15ml of vg, now they have 30ml of grape with the usual flavor percentage in 9mg strength. It is a way to save money, gradually work down nic levels, etc etc.