http://www.uscis.gov/sites/default/files/USCIS/Resources/F4en.pdf
Says debit or credit card or US bank account. Do you not have a debit card? (Many bank ATM cards are also debit cards)
If not, your other alternative then is to pay after you enter, but do that as soon as you can to cut down on the delay in processing your green card.
Please how long does it take for me to get my SSN. Does it come together with GC?