I think a simple UI change could be in order. If you select "Launch", and your credit total < rebuy + cargo insurance, then you get a popup that says something along the lines of "You're about to launch without the credits to cover yourself should the worst happen. Are you sure?" and get a confirmation that you want to launch uninsured.
Oh my! ..and add are you REALLY sure?? ..and REALLY, REALLY sure??? And add a disclaimer to not post a sympathy thread of the forums should you foolishly decline Ins. and go poof.