I noticed that each time I get my haircut reservation to the day after the day conversation takes place, I hear the person on the other end changing the way he or she says 明日 from あした to あす or みょうにち, when he or she makes the final confirmation of reservation day. It happens pretty much every time I do the reservation over the phone, while I struggle to remember hearing people using みょうにち in normal life at all.
It sounds like it is done intentionally as it happens every time and not limited to only one particular person, so I think it is either a company guideline or some general rule they are following.
The only possible explanation I can see is that it is done to avoid misunderstanding. I know that 一 and 七, as well as 四月 and 七月 might be mistaken for each other when spoken over the phone and therefore might require clarification by using alternative readings. But using alternative readings for あした baffles me as I can't think of what あした might sound similar to given the context of the conversation.