The Feds for the most part are good people and will approach you in a professional manner.
And for the most part will only get nasty with you if you decide to rub them the wrong way. After all, they are only doing their job. And part of their job is to make your life miserable, if they choose to.
I used to deal with these guys ( U.S. Customs and Border Protection or CBP, Canada Customs ) on regular basis and if they said they need to search my truck or send my truck to the X-ray my answer to them was "I know the drill".
They would laugh and send me on my way. When you have nothing to hide then you have nothing to worry about.
Point is,,,
You can choose to be polite and let that one or two USDA agents come and do their job or you can be that tough dude have a couple dozen more armed to their teeth and tear your house up. Either way, they will get their trophy. That trophy can either be your plant material or your rear end.
My 2 cents.