Depends on your USPS office, usually handoff is the next day. Keep in mind that UPS partners with USPS on their UPS SurePost® offering, where USPS delivers parcels the “last mile”, so that low intensity service areas (for UPS) don’t need UPS’s full attention. USPS is probably there every day. So they already have a protocol.

Anyone had this happen

Yes. Threw the wrong parcel into the blue collection box after hours, then cussed for a minute.

and the package got lost?

No. it worked out fine.

long time ago, I got the package back by tomorrow
but now, some body told me , they will pass it to UPS

Good advice here. I was also told by our USPS that they forward UPS when the occasional accident happens. They made it sound like an everyday thing.

Thanks for the laugh. Im glad my paranoia of doing this is warranted.

Update. They sent it back to me. Took about 2 weeks to get

My experience has been,…if I drop a USPS at UPS by mistake…UPS will get it to USPS the same day usually even and tell me when I come in the next day even that I messed up…lol. If I drop a UPS package at USPS…its gone. Its like they are bitter and trash it or something…I dont know what happens to it. lol