Unfortunately Apple's intention is that you would not hear about an alternative way to pay for the subscription and that the alternative way is cheaper.
I mean if you partnered with Target to sell your shampoo I think they would be rightfully pissed if the bottles you sent them to put down said something like "Buy it on our website instead for 30% cheaper!"
I imagine that if a consumer sees a product at Target, they have an idea that the product might be cheaper somewhere else. However, do consumers have an idea that if they seen an in-app purchase that the exact same thing might actually be on sale somewhere else and that it has a different price?
If the app says that "One year HEY subscription" is $142 and the app is from HEY itself, what reason is there to believe that it's not just the plain old price of HEY?
