The important thing is to give the IRS an address that you can be reached at, in case they need to mail you anything.
If you are talking about where to mail your paper return to, please see Where to File. But can't you e-file?
To be honest, I don't know if it matters what you tell pay1040.com, because I don't know what difference it makes...you are paying, after all.
I am not sure that I understand what you are asking.
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