Call them back and make them fix it.
They offered lifetime for $299.99 and you accepted.
Offer and acceptance.
Anyone who ever watched "Paper Chase" or any other lawyer show can tell you.
That's a legally binding contract.
They didn't fulfill their end of it until you pointed out to them that they hadn't fulfilled their end of it. By that time the price of lifetime had changed, which is where they made the mistake, but the change doesn't apply to you, because you had a legally binding contract for lifetime at $299.99.
They owe you a refund of $100 and of all of the monthly charges wrongly imposed since you accepted their offer.