Printing, signing & mailing
Paying the USCIS filing fee
USCIS charges a filing fee for the I-539. The exact amount and accepted methods are shown on your Package screen (always taken from the official source, never quoted from memory). Three ways to pay when filing by mail: (1) a check or money order payable to "U.S. Department of Homeland Security"; (2) Form G-1450 to charge a credit/debit card; (3) Form G-1650 to debit a U.S. bank account. In this app you can fill G-1450/G-1650 securely — the card or bank numbers are encrypted and never appear in the chat.
What if…
Which is better, G-1450 or G-1650?
They are just different rails: G-1450 charges a credit or debit card; G-1650 debits a U.S. bank account (your bank may need to allow DHS ACH debits). Pick whichever you have. There is no advantage to USCIS either way.
Official sources
This is general information, not legal advice.