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I do not believe you qualify for a stimulus payment for your spouse who died in 2018.

IRS FAQ under Will a deceased taxpayer receive the payment? states:

A payment won’t be issued to someone who has died before January 1, 2020. If you filed a joint return in 2019 and your spouse died before January 1, 2020, you won’t receive a $600 payment for your deceased spouse, but you will still be issued up to $600 for you and $600 for any qualifying children, if all other eligibility criteria are met.

IRS FAQ under What should I do to return an Economic Impact Payment?

A1. You should return the payment as described below.

If the payment was a paper check:

1. Write "Void" in the endorsement section on the back of the check.
2. Mail the voided Treasury check immediately to the appropriate IRS location listed below.
3. Don't staple, bend, or paper clip the check.
4. Include a brief explanation stating the reason for returning the check. 

If the payment was a paper check and you have cashed it, or if the payment was a direct deposit:

1. Submit a personal check, money order, etc., immediately to the appropriate IRS location listed below.
2. Write on the check/money order made payable to “U.S. Treasury” and write 2020EIP, and the taxpayer identification number (social security number, or individual taxpayer identification number) of the recipient of the check.
3. Include a brief explanation of the reason for returning the EIP.