Line 30 is only if you didn't get the full amount or qualify for more. The 6475 letters were telling you what the IRS sent you. You should have received the total of both for 876.40 deposit. Double check your bank statements again and Check your IRS account (both accounts if married)
Your Online Account | Internal Revenue Service
If the IRS thinks they sent it to you but you didn't get it request a trace for a missing third Stimulus payment
https://www.irs.gov/newsroom/questions-and-answers-about-the-third-economic-impact-payment-topic-j-p...
