May I setup an error message if the customer does not fill in a required piece of text?|
Yes, you can.
For example if your text input was:
<input type="text" name="VARName">
you could put the following into your form:
<input type="hidden" name="VARName_undefined" value="ERROR: please enter your name">
If the customer forgot to enter their name the cart would display a message informing them of the fact.