As far as I know, Nirvana Seed Company is pretty reliable. People I know use them. Whether it's thru sites like RIU, I never bothered to ask, but it seems to be a good company.
They have been around since I started growing nearly 20 yrs ago, so that has to count for something. Googling now seems to pull positive reviews for them. Solid genetics.
As far as convincing on the safety of mail ordering.....I have never done it. I only either did bag seed, then clone the cream of the crop (pun intended), or get clones from sources that I know, which get their stuff from reliable companies such as Nirvana.
I have even heard of them going so far as to replace seeds that never arrived.
Don't know what else to suggest on this.....anyone else???? Interested myself.