For purposes of this section, retroactive premium rate increase shall mean any premium rate increase based upon an actual approved rate which is assessed to a subscriber after any payment has been made by said subscriber based upon an estimated premium rate for such period of coverage.
A nonprofit hospital service corporation governed by this chapter which offers medicare supplemental individual or group coverage shall, in the event of a retroactive premium rate increase for such coverage, allocate such increase among the remaining outstanding premium payments due; provided, however, that no finance charge or additional cost may be charged as a result of such allocation.