In answer to your questions:
1) By default, the "Apply now" button emails the employer who posted the job, so if you wanted to change this it would require some customization of the default theme on your part.
2) Relisting of jobs is under the same conditions of the original listing, so no discount options are available by default. There are no settings specific to users posting habits, so discounted rates based on number of jobs already posted is not possible by default. Specifics of the Job Pack settings can be seen on the
JobRoller admin demo.
3) The job post is paid for prior to being published, so if the credit card expires after it has already been paid for then this would have no effect. If it is already expired before attempting a purchase then PayPal won't complete the transaction. Thanks.