"OUR letter from the IRS" said they sent 2100 is not right ... the IRS sent each spouse their own letter for 1/2 the amount they were sent. So if one letter said 2100 then the other letter you did not get also said 2100 thus 4200 combined. This would mean you had 3 people listed on the 2020 or 2019 return that they used to send out the third payment on but if you were only eligible for 2800 on the 2021 return then you get to keep the excess. What you did is only entered the 2100 instead of the 4200 or even just 2800 so you incorrectly got a 700 credit on line 30 that the IRS disagrees with. So review your bank records for the IRS EIP 3 deposit starting in March of 2021 ... it may not have come in one deposit.
