We use a few custom flat rates for both USPS and FedEx. We do this because we like to control the name of the service (and the price). It also allows us to avoid any carrier API issues during checkout.
Anyway, with the launch of Shop Promise we noticed that our custom FedEx 2Day rate was generating a promised delivery date of Saturday for orders placed on Thursday. As I'm sure you've already guessed, that isn't happening. 😄
Before shutting off Shop Promise, I asked support:
The way Shop Promise determines the predicted delivery dates is by using fulfillment data provided by the merchant, such as the location where the order is processed, tracking IDs, and the delivery destination. We also consider historical data from the shop's shipping and delivery records.
So in other words, it guesses based on the history which would be acceptable in this case except for packages that calculate to a Saturday delivery on this method. In this case, the package would deliver on Monday. We have lot of language on our site about this and pretty much no one is ever confused.
The rep passed along all of this information to the development team who will no doubt add some logic. No idea when but until that shiny day comes we'll have Promise turned off. Bit of a disappointment though as it is a cool feature.