You should rename field_email to field_basic_cart_email to avoid conflict.
For example :
I've field_email on my site with the type of email (email module)
When I install basic cart which comes with field_email with type of text, It will try to change existing field type.
I got this error FieldException: Cannot change an existing field's type. in field_update_field() (line 234 of on every field_update_field() call.