Outgoing mail can be put inside your curbside USPS mail box.
To send outgoing mail through your USPS curbside mail box you just put the mail into the mail box and then put the flag up.
When you put the flag up for the mailbox it alerts to the USPS mail carrier that there is outgoing mail in the mailbox.
Then when the USPS mail carrier sees the flag up on the mailbox they will stop and get the mail from your mail box and then they will put the flag back down.
If you don't put the mailbox flag up the USPS mail carrier will not know to pick up the mail from the box.
So just make sure you put the flag up and they will pick the mail up.
I mail outgoing mail in my USPS mail box at the curb all the time and they've always picked it up.