Maybe because you outsource fulfillment to a legitimate company, and they will find it strange to send empty envelopes?

So instead you give them seeds to ship which probably cost less than the envelope itself.

I wonder if package weight is available in the tracking information that Amazon or other seller service can access?

Gotta match the weight of the package to the product weight.

That’s what I don’t get. Maybe if a customer received an empty envelope they would think something is up and report it? By sending a token item the receiver thinks it is a gift? But why not a postcard then which is easier to include?

Part of me wants to think this is an elaborate prank to see what happens when hundreds of people plant mutant seeds.