If you filed a joint return you are both liable for any additional tax and interest on the return. However only the interest amount of $7 is due once (not twice).
Just write a check (or money order if you wish -but not required) for the $7 and mail it in and be done with it.
Make sure you write the return and year it is for on the check so it gets correctly applied along with the stub from the letter you got from IRS. So if it was 2017, write 1040 12/2017 on the check.
