I filed electronically and selected the pay by mail. The post offices in my area closed/ Is there a way to get the post for today or submit it electronically?

yes, you can switch to efile if there are no open post offices in your area.

https://ttlc.intuit.com/replies/7191107

If you already filed electronically and your post offices are closed- you can go online and pay your balance due. 

Visit this IRS website for more information https://www.irs.gov/payments