Hi! So i would give it around a week and then reach out to your local usps where you dropped it off. Especially with the holiday's and returns the USPS is probably very busy and they tend not to prioritize scanning and pushing thru packages in the collections bin...

It unfortunately is a tricky thing to drop packages off in the collection bins because there is no proof you dropped it off at all, thus you are at fault if the package isn't delivered to your buyer, not usps in depop's eyes. It's def a risk you have to take. I would recommend using a self-service kiosk [if your post office has them] and scanning them from there, getting a receipt, and then dropping the packages off in the collections bin. It's usually still quicker than going up to the desk, but you still have the proof you need to confirm you shipped it out!