In the meantime, you can pay either Apple or Fastmail for this feature as part of their larger product offerings. Maybe there are other paid services which offer this as a feature. These are just the ones I'm aware of.
I have read the FAQs page but I am not seeing an answer to a question just yet: can one lock the 0.99 USD limited time discount forever?
I think the honest answer there is that there is no way to promise that (who knows who's managing the project in the future). But I do know that the folks currently on the team are planning that, when we do raise the prices, we only do so for new subscribers (but again, this is not a promise, just my personal expectation).