Use Rules to give users a discount on their order after a few days (Commerce)
I haven't done anything date related, but I have made some rules to apply discount based on various specifications. You should checkout the Commerce Discount module, it integrates in with rules. 

So you would just need to create a rule to add a scheduler item for each of the time periods that you are interested in when the user adds an item to their cart, or whatever event you need.

Then when each scheduled item goes off, you would have a rule to apply a discount to their order, and do whatever else you want (maybe notify them by email).

Then a final rule that would delete any additional scheduled items once the order is marked at complete. 

You should be able to accomplish all this with a few rules.