I'm staying in but what if the mailman has covid19, how do I sanitize the mail?

You can still open the letter but make sure you wash the hands right after you touch them. 

Sick mailman cannot work...clorox,  lysol spray will do...

You can't sanitize your mail. However, you can wear gloves when you pick it up and read it. Than, wash your hands.

spray it with dettol or lysol spray

Chances are you won't catch it that way...

and if you do, half the neighbourhood will have it too

wipe it over with a sanitized cloth before you touch it , was your hands with soap and water and dry off using a paper towel. Then open the letter and get the virus from who ever wrote it who has the virus and sneezed on the paper before putting it in the envelope. You worry too much about stupid things

dont worry about it, they'll send another mailman if that one is sick

Wear disposable gloves (you can buy them in boxes at the supermarket, then maybe spray the envelope/package with tinned spray disinfectant. Or, just wash your hands with soap after handling the mail. You could use kitchen tongs to grab the envelope out of the letterbox. 

Use a UV light.

Handle it with rubber gloves.