Customs letters do NOT necessarily represent an LEO investigation. You may not have even been the person who ordered the seeds. You could have been on the "receiving" end of a prank, or even the person someone was shipping to... you could have been the "buddies" house. There are a lot of variables. Hell... too many to think of really, but precautions are always to be taken, HOWEVER... by the time customs has your order, if LEO was interested, they would already be on you before you had the opportunity to dismantle your grow. They wouldn't send you the letter if they were to investigate. Remember, you don't know how long customs or LEO has known about your seed confiscation, etc.
BUT to make you even more paranoid, anyone who orders seeds NO ONE knows for sure that customs didn't let it "slide" by on purpose in an attempt to trap those who grow. So if you ordered seeds, there is still a possibility LEOs are watching by "slipped" seeds and informants.
ALWAYS order small amounts of seeds. LARGE boxes of seeds immediately flags a prompt for further investigations.
PO Boxes are BETTER, but NOT best... there is NO "BEST" option.
pO Boxes still require a valid drivers license to open one, and you MIGHT be able to find a cash only type po box somewhere in that wont ask for ID, but if the LEO wants to investigate, they'd just stake it out anyway.
BUT yes, what I do is I started by growing tomatoes indoors in my hydro systems. I typically grow mj in soil for quicker movement or destruction. But anyway, you should always have a tomato plant growing amongst your MJ, this way, if you do suspect LEO, simply remove the MJ, and leave the alrady in progress tomato grow going. This way if they bust down the door because of them doing a infrared hunt for lights and hotspots, they will find a single little tomato plant growing! All that money and man power they've just wasted! Then... turn around and sue their behinds for a plethora of things!