In our experience Northlink are super quick to refund any monies due. In fact they immediately pay it back into your bank account. Also, the price of fares do not depend on how early they are booked but rather on what season they are booked.
I've often wondered why Northlink don't run a sort of "loyalty scheme" or some such thing for Shetlanders who frequently travel with them. Such a scheme might be of interest to cash strapped students or the like as it would make frequent visits home so much more attractive. Another idea is that they could have last minute deals for empty cabins in less busy seasons. That could be a real money spinner for them (as the boat goes anyway!!) and very attractive to homesick young people. What about it Northlink? Better a full winter ship full of youngsters than an almost empty ship full of crew!