you would need a quick turn around of when you get their money and sending the seeds. tell them never to send money to you with order form as recorded delivery and send them a email when you get the money.
send the seeds in cotton wool or bubble rap to prevent damage to increase customer trust.
send weed via regular mail same as with the seeds and put in a cardboard box to make a rigid shape preventing damage or detection no other security measures need to be taken as regular mail is not checked even when sending intanationaly. but i would recomend staying within in you country. add correct amount of stamps and put in a post box.