Would an ounce or few of MHRB fit in the small 3x5.5 inch PO boxes? I'm pretty sure about the Rue seeds. I would like to know before I spend my money. Thank you.

I bet 50-75g mimosa and an ounce of rue could fit into one of those. Though with packages like that, it's not infrequent for the post office to keep them behind the counter anyway, on principle.

I had a PO box many years ago. If the item is too big they will put a claim ticket in it for you to pickup inside.

Mine just puts a key in the box to open a one-time larger box. The USPS knows how to handle it.

Just what I needed to know!