how can i get the humblebundle if i have cards to pay for it?

SteamTrades
You cannot use steam wallet credits to buy outside steam, so you would have to trade someone your cards in return for them buying you the bundle.

SteamTrades. No idea what your cards are worth since you didn't name them. Check the prices on market.

Sell your cards then buy games to trade.

Sell them all, buy keys, find a trader. 5 or 6 keys sounds about right, unless the average, or key prices have changed since I last looked a few days ago.

Trade cards for tf2 keys, I saw one guy trades 3 tf2 keys for 1 BTA bundle, because find someone who will buy it to you for cards is difficult I think. Also the card "worth" determines by how much the seller receives and not for how much is on the market for the buyer. 1 set of cards normally cost around 1 tf2 key, depending on game different amount of cards is needed to make a set.

lol The Zeppelin of Consequence (Foil Trading Card)

this card is not worth 8USD, you wont sell it never, or maybe once a month with a huge luck. Check sale history, 1 USD average.

it would be hard to find someone, who will buy you BTA bundle even for all of your cards. Better use TF2 keys for this.

Other cards in the same set typically go for less than $1 - that card's price is only high because somebody purchased 9 of them a few days ago in order to try and drive up the prices, and most buyers seem smart enough to notice that. With that said - assuming it's a genuine transaction - there was one sold for $8, and if you're lucky then someone else might be desperate enough for that card to pay that much, but it's not the amount you should expect to get.

One of the most subtle trade threads I've seen!

WHERE IS THE REPORT/PANIC BUTTON?!