When you receive your stamped passport you have to go to USCIS and pay the fee before you enter the States (I think its technically possible to enter and then pay, but advised against).
You receive an email from "ELISdonotreply@uscis.dhs.gov" as a receipt and in that email is the following:
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Application Name: USCIS Immigrant Fee
Pay.Gov Tracking ID: *************
Agency Tracking ID: ******************
Transaction Type: Credit Card
Transaction Amount: $220.00
Payment Date: Oct 24, 2023
Transaction Date: Oct 24, 2023 03:42:16 AM
Your payment was applied to the following case(s):
#*#*#*#*#*#*#*#*#*#
THIS IS AN AUTOMATED MESSAGE. PLEASE DO NOT REPLY.
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The number you use in https://egov.uscis.gov/ for tracking is the #*#*#*#*#*#*#*#*#*# number that Ive redacted here.
For the last two months when I used that site It just told me the date of payment. However it recently changed and said they had processed it. 1 and a half weeks later and its changed again to:
"On January 23, 2024, we mailed your new card for your Form OS155A, Immigrant Visa and Alien Registration, Receipt Number #*#*#*#*#*#*#*#*#*#, to the address you gave us. If you do not receive your card by February 7, 2024, please go to www.uscis.gov/e-request to request that we send your new card to you. If you move, go to www.uscis.gov/addresschange to give us your new mailing address."