At the bottom of the label there is a place for you to sign and write in the date.
I've read of people forgetting and it going through fine and of others who have had it returned to them for not having it signed and dated. It depends on how closely your PO is checking?
I've had two different postal workers tell me two different things about signing and dating (shocking, eh?). One told me it has to be signed and dated and the other said because the labels are generated online through a "known shipper" --you should sign and date, but it isn't the end of the world if you don't.
I go with signing and dating because obviously different clerks have different views on it.
Hopefully, yours will go through fine.
Edit to add: The customs number can work like a tracking number while the package is in the USA. You can see if the package is "moving" IF it is being scanned. If I remember right, however, a package can be stopped and returned to sender at any point in transit if a PO decides something is wrong with it (wrong postage, not the right paperwork, etc).