Votes are counted on local levels. Are ballots not mailed in to the town level? I see no reason why every town hall can’t have a secure drop box by its front door.
The drop boxes are NOT just “boxes” that you deposit things into unsupervised. You have to contact a person manning the drop location and prove you are the person listed on the ballot envelope, since only the person voting may drop off the ballot and no one else. Makes sense, since “ballot harvesting” is a real problem in many locations.