just about everything shipped these days comes packaged with a silica pack (boots, batteries, electronics, etc etc.) - you can save them and reuse them no problem - ask your friends to save them too.. (they are great in tackle boxes, ammo boxes, tool boxes etc. as well)
it is ESSENTIAL to be sure your seeds are dry dry dry before you package them..in late summer/fall i have plates all around my house on books with seeds sitting on scraps of parchment...patience is key - 2 weeks seems safe for internal seeds (peppers, tomatoes, squash, cukes etc.), herbs you harvest dry as a bone on a dry afternoon - them store the stalks til dry dry dry.. beans harvest crispy.. then dry more
did i say dry?? you can't rush the process .. putting them on plates gives insects a chance to skedaddle as well
to store the seeds, i use coin envelopes ..
https://www.staples.ca/collections/coin-envelopes-8902
they are pricey but work great a they are tough and breathe (i would advise against airtight containers) use a paper clip to seal them up as opposed to to wetting the glue flap you can re-use them until there is no space to write... i store the envelopes in plastic shoe boxes, with silica and those go up in a wood cabinet in a dry place in the house that has a door that closes (a kitchen cupboard up high would work well so long as mice can't get in)..
not much more to it but all of the above really does work reliably
good luck - cheers!