Mid/Endgame - How to sell more / XP more ?

[noty]

Hey there, I've progressed to the 5th area and I'm on my way to unlock the last one.
I produce a ~$720 coke (8 mix) and some meth. I give my dealers both products but I sell only the coke.
However, no customer has ever ordered more than 1 and they bail out when I try to counter-offer. On a daily basis I have like 10-12 deals and that's getting slow to gain XP and level up.

Should I also make just plain simple OG Kush and let people order this so I can counter-offer ? Will I make more individual sales ?

I feel stuck in this loop which is getting more and more tedious given the constant resuplying and laundering. The worst part is that my product keep stacking up, so I feel stuck and don't know what to do to progress.

Any hint or idea to improve my game ? :)
Thanks