A single payment will unlock everything set at that price or below.
For example, right now, you can get version 0.117 and the "Sneakily Humping" audio file for free, you can pay anything between $2.40 and $4.79 to unlock the private release updates, or you can permanently unlock everything with any payment>= $4.80.
The same principle applies to any other game on this site which gives bonuses for payments here: this isn't Patreon, you don't have to pay multiple times for the same content.
It shouldn't change as far as I'm aware (I paid $6 a year ago, and I have gained access to every new update and every audio log by just accessing the Download tab next to the "You have paid X amount of monies" notice at the top of the game page).
I assume that if a higher price item ever gets added, you could unlock it by just paying the difference between your previous purchase price and the price of the new tier, but as I said, the prices on this game haven't really changed in over a year.
