Compelled to respond a second time.
Again, I completely side with OP. Yet, I find it hard to be 'completely unaware' of something that is literally at the very top of the website where it reads "FREE E JUICE ON PURCHASES OVER $15."
Yet, you say your order came to $15 before taxes. Well, that (and shipping) is something you purchased. I mean if "rules are rules" then hey let's get all technical on what the consumer purchased.
Moreover, you had a comment and unless their database is primitive, it would show you are new customer. Without the comment, I can see reason to try and get away with the exclusion. With the comment, I'd send an email at very least, before order is shipped, that explains the matter. I mean, I'd do this if I cared about customer service.
But still seems a whole lot easier to just include the requested sample and move onto next order.
Added: After reviewing the MBV policy page for the "free e-juice," it makes clear the way it is calculated and that it is 12 mg/ml at 80/20 PG/VG split. Knowing that, if I ordered $40 from them, I'd be adding in comment saying, "please do not send me a free bottle. Thank you."