If I buy a Humble Bundle not beating the average, can I later pay more to beat it and get the extra games?

So lets say, I buy the Humble Bundle with Android 7 (the current one at this moment), but I don't pay above the average, I obviously won't get the extra games.

But can I go back and pay more later to get the extra games if I already made a purchase? E.g. if I now pay $1, but later the average price is $4, will I only need to pay the remaining $3, or would I need to pay the full $4 on top of the $1?

Or will I need to make another purchase?

Also, since now for the second tier it is mandatory to have paid above the average, will I be able to pay more in the second week if I want the second tier games?

All this is because at the moment, I am not interested in the current extra games (Bards Tale, and Worms Reloaded), but if in the second week they add better games, I would like to be able to get them.

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.

I can verify that after the sale goes away you can add up to the average and still receive the codes for the extra games.

It seems that as long as the link is available to increase your amount and the info below your paid amount shows the 'Make sure your total is at least $6.00 to unlock The Alien Breed Trilogy and Worms Revolution Gold.' which is the case with the Team17 bundle I bought you should qualify for the bonus games, but I would assume it's only valid for as long as the link is available, which seems to be the 30 days quoted.