In order to prevent  overlapping discounts and possible order corruption, it is important to set the option for the number of days that an order can be edited in the OpenCart 1.5 Administrator. The following tutorial walks you through how to set the number days that an order can be edited.  Note that the period value is set to DAYS.

How to change the the number of days that an order can be edited

1. Login to the Administration Dashboard as an Administrator
2. Hover over the menu bar where it says SYSTEM and then click on SETTINGS in the drop-down menu
3. Select the STORE that you wish to edit and then click on EDIT on the far right to proceed
4. Click on the TAB that says OPTION, then scroll down until you see the option labeled ORDER EDITING as shown below:

   

5. Change the value to the number of days that you wish to use.  Keep in mind how often your prices vary for a product while making this change.
6. Select SAVE in the upper right hand corner to save the number of days you have selected.

While the effect of this change isn't something that you will immediately see, it is a required change since it prevents your orders from being changed after an order has been completed.  As noted in the fine print by OpenCart above, it may cause corruption of the order if there was conflicting changes.