check your contract - if it says as much then they can do that - and it is your job presumably to know whats on your van.
i would suggest perhaps writing to them explaining the circumstances - the medication, being unwell, long day etc, but continuing so as not to let them down and apologise profusely, promise it wont happen again, and ask them if on this occasion they would give you another chance
they can only say no.
be careful not to sound accusatory, defensive and whiney with comments like "you knew i was ill", and "how would you like it if you had done a 12 hour day etc" or "its only a package" etc ... just be apologetic and humble...