I have attached a simple mod that will do what you want. It will display a bordered box (whose border is created using your native stylevars) into which a message will be put:
To set the message you wish to be displayed, edit the following phrase:
Note: In the screenshot, the wrong product is shown, but I have corrected that, the name of the phrase is the same.
I used a phrase so that you can use translations if needed.
You can format your message using HTML. If you aren't familiar with using HTML, then let me know what you want the reminder message to look like, and I will write the code for you.
Just import the attached .XML file...there are no settings, once you import it, it is active and working. If you want any added features or changes, please feel free to let me know.