I don't see us getting to the point of having to use a mfrb, that would be a ton of seeds. Wish they had a size between sfrb and mfrb, it's convenient that the flat rate boxes come with tracking # and $50 insurance. If we do have to switch to a bigger box everyone will just have to remember to make sure and get tracking. But we'll cross that bridge when we get to it. I'll do some investigating, I believe one of the other old trains had to switch to a bigger box and it was cheaper than the sfrb but again have to add tracking not really worried about insurance.