I'm not sure what I may have done to affect this, but it seems now whenever a user selects the offline option and proceeds to click through as instructed (i.e. the pink button & pop up message) followed by a pop up of the payment slip, nothing else happens.

I seem to recall that the page redirected to a confirmation page but don't recall anymore. If not is there a way to set this up
The way it seems now, the user may be left wondering what's next.

By the way, I have set up a mailer that will be sent after a cron fires with payment instructions but until the cron kicks in the may not get anything fast enough to assure them all is well.

That's a good option. I can put some wording on the payment slip with further instructions.