When you have obtained a visa number and the U.S. consulate has granted you lawful permanent resident status, you will will need to pay an immigrant fee.
You pay this $220 fee online. USCIS recommends that you to pay this fee after you receive your immigrant visa packet from the U.S. Embassy or consulate abroad (including Canada and Mexico) and before you depart for the United States.
Paying before you depart for the United States will ensure that you receive your Permanent Resident Card (green card) without delay. More information about the fee can be found on the USCIS website at www.uscis.gov/immigrantfee.