Welcome to the community!
Was the cost $20 after bill credit? It's a common misconception that the monthly cost of the phone was reduced. However, that was technically not the case. The device cost shown on the website was calcuated after bill credit. The monthly cost of the phone remained the same but customers were getting a credit on their bill to offset the cost of the phone.
If your promotion was after bill credit, your account should have a credit of $19.95. From the information you provided, that would calculate to the $20 advertised after bill credit. Are you receiving a bill credit? You should be able to view the details of your bill via My Account --> View & Manage Bill. From there, you should be able to Print and Save a PDF version of your bill. Alternatively, you should also be able to view the details online via the Monthly Charges, Usage Summary, and Other Charges and Credits links.
Hope this helps 😀