Libby, for my swap and future swaps I set a limit of 100 packs of seeds. That is the approximate amount of seeds that will fit in a small flat rate box. If the participants pay $6.00 priority mail to send their seed to you and another $6.00 for return postage, then they are paying a total of $12.00 shipping. Assuming seeds cost at least $3.00 per pack, then 100 packs of seeds would be a $300 value for only $12 dollars shipping.
If you don't want to limit the number of seeds picked then set a sliding scale. $6.00 for the first 100 packs picked. If they go over that then they need to add an additional $6.00. If someone goes just slightly over, say 110 packs and you find you can get them all in a small box then you can just refund the extra.
