I get it, I really do. Along with building sites, I run a brick and mortar retail business. It really is about managing expectations. Given your email will go to a majority of subscribers (I'm hoping that is a safe assumption, and if not, just consider those who do subscribe as your "valued above all" users), you set up disappointment by not addressing this on the front end.
If I get an email that leads me to believe I will get a discount, I will be so much more disappointed after clicking through and trying to get the discount and finding I don't get it because I already have paid you, than if I understand that this discount is to attract new customers on the front end. One thing I would strongly suggest is separate your active subscribers from prospective clients. I do this to make sure I don't upset those who already pay me retail. I really like your company and your staff and want you to do great.