Plusnet's billing system has been a mystery to many since their "upgrade" at the end of 2018, after 2 years of billing and contract dates issues I left in December 2020 after 11 years of partonage.
Be aware that they are notorious for passing any perceived outstanding "debt" to third part debt collection agencies which can be very troublesome to resolve and could effect your credit rating, although, for such a small amount would doubt if this would be practical but with Plusnet who knows?
My choice would be just pay the £1.39, from long experience probably a waste of time trying to find out exactly what it's for and certainly not worth the hassle, obviously, the decision is yours.