The data you enter into FileMaker does not simply disappear on it's own. Some action on the part of the user or a script must delete it.
It's possible that the data is still there, but are not displayed once the current date advances to the new day. You should have a layout based directly on the portal's table. Switch to that layout and use Show All Records to see what data is present. I suspect that all your records are still there and you simply need to design an effective way to find and display it. The details of that will depend on the design of your database.
Yes, the data is still all there, and I know that its an issue with how to display it.
Thanks for making it clearer, I have a good idea now of the solution.
I'd create a new record for each delivery day with a portal that lists that day's deliveries. That way, researching a past day is as simple as finding the record with that date.