What counts as proof of mailing for Colorado State income tax?

[sandrawa]

What counts as proof of mailing for Colorado State income tax? I don't have to pay taxes but I need to mail in to claim refund, what counts as proof of mailing? They said don't send certified mail because that wil ldelay things 

[JohnW152]

The only way to get proof of mailing is sending your return by certified mail.

You may be looking at Colorado's Contact Us By Mail page.  The delay time when sending a document by certified mail is minimal, generally no more than one day.

[sandrawa]

I don't actually owe money to state of colorado would mailing it usps priority mail suffice? I am just claiming refund

[SusanY1]

USPS priority mail with tracking would be sufficient proof that the return was delivered and will also report the mailing (post mark) date.