the us post office offers many different things for mailing...flat rate envelopes and flat rate boxes (in more than one size) that can really be helpful when sending stuff heavy stuff or a lot of things...it's a flat rate no matter WHAT the packages weigh!!
sending to other countries is a whole different issue. even to canada now there are restrictions. EVERYTHING that is sent out of the country (the us) is supposed to have a claims form attached.
and remember, there ARE restrictions on what seeds
stuff can be sent in/out of our country and each other country has thier own restrictions too!!
here's a link to the usps info for #139.2 Plants mailing restriction
for things sent out of the us.
and here's one for the individual country info
for what's allowed to be sent in to those countries.
it's a really iffy thing, these days, to be sending plants
to/from and even seeds