humble-bundle
I bought Humble Indie Bundle 9, beating the average, and got all the games.
The page states that there are more games to come:
But the average is now above what I paid earlier. Will I get the additional games, or will I have to pay more, to reach the future average price?
You can add the games to your account under Linux, but none of the games in Humble Bundle 7 support the Steam on Linux beta yet.
Yes, you can do that. Go to the specific page of your humble bundle, not your account. There you will see a link to increase your purchase.
Clicking this link will open the payment options:
As you can see, it already "knows" how much you paid and you can top it up to any amount you wish or need to beat the average and after paying you will find the new keys below on the same page.
Best Answer
Yes, if you pay above the current average, or increase your payment to above the current average at any point, you will receive any additional games added to the 'beat the average' tier.
If you look at the text on the image you linked:
This means you'll get any new games when they are added.
Extra Source: I've done it.