I really like them and keep one in my car for similar purposes to what you mentioned.
I'm sure you know, but they work via bluetooth so you can't track them unless either your phone is nearish it, or if you report it missing on the 'Find My' app then you can also find it if someone elses apple device is near it. But if you do the latter, bear in mind others may be alerted to the missing device. Also if somewhere along the way the delivery driver is using an iphone, they'll get an alert they're being followed by your airtag as the device does this if it's following someone else and not in the vicinity of your phone. So on balance I'm not sure it's ideal for the purpose of tracking post.