Inverness to Glasgow Bus

That is for Scottish seniors over 60 only. They can use all buses, local and long distance for free.
The £2 is the optional seat reservation fee.
On boarding the bus you have to produce the Scottish Entitlement Card to prove eligibility.
Even the English don't qualify.

So for you it isn't true.

Citylink haven't opened booking for June yet, but there will be a fares rise before then due to the impending changes in free pass holder reimbursement rates, which have already meant a number of local bus operators have put fares up.

Presently, the pre-booked Adult Single rate is £25.80 Inverness Bus Station to Glasgow Buchanan Street Bus Station.

There are also trains from Inverness to Glasgow, both direct and with a simple change at Perth or Stirling to Glasgow Queen Street. Fares start at £20.90 booked in advance, (going up to £21.70 in April) and the direct trains are slightly quicker than the Citylink coaches.

The Inverness to Glasgow buses- which are hourly- are a codeshare between Scottish Citylink and Megabus, with Megabus being the technical lead operator, although with a pooled fleet (Megabus, Citylink and Parks of Hamilton).
It is always cheaper to book through Megabus rather than Citylink.
Booked through Megabus the fare is around £13 + booking fee. Even for travel today Megabus are a bit cheaper. They only open for bookings 6 to 8 weeks before travel.
Flixbus also run a few services a day.
Lead in fares for them are £7.48 + booking fee. Even for immediate travel today around £17.

Although it is unlikely before June there is a 3rd operator about to enter the market, which shows how strong the demand is for bus travel between the two cities.