How can I configure my Shipping Manager to propose a delivery date?

The following configuration can be found in your Shipping Manager under the admin section/delivery setting.

-2- "Display the delivery date": By checking this option, a date picker will be shown in the front-end iframe
-3- "Number of visible days": The number of possible delivery days shown to the customer by the date picker the front-end iframe. 
This number ranges from 2 to 7 days. "Do not specify" deactivates this option.
-4- "Days before delivery + one day": The number of days you as shop-owner need to prepare a shipment + one day for bpost to make the delivery. The resulting number will be used to calculate the estimated date of delivery.
This number ranges from 1 to 9 days
-5- "Last possible order on your webshop": Your cut-off time for orders on your webshop for a given day. All orders after this time are assumed to be be treated the next day.

When do I need which settings?

This is explained in the following article

In any case, if this information should not answer your questions,do not hesitate to ask us!