Hi Karin, if you need to show an error message during Shopping Cart creation you willl need to implement badi BBP_DOC_CHECK_BADI, and maybe create an RFC Function Module to check in ECC ir the minimum order value was reached or not and show the error message.
In order to control the following document you will need to implement badi BBP_TARGET_OBJTYPE.
Find bellow a sample BBP_TARGET_OBJTYPE - Code Approach.
Hope This Help.
Celeste