In a farm share (CSA, community-supported agriculture), at the beginning of a season you subscribe to and pay for a share, and then you receive regular deliveries of whatever food those farms produced that's ready. Some CSAs send out preview email a few days before each box is delivered, telling you what you're getting.
This made me wonder about cases where the box includes chameitz during Pesach. If you don't own the food until you pick it up, then you could work around problems by having a non-Jewish friend as a partner in the share, and that person takes anything problematic before you touch anything. On the other hand, if you own the food as soon as the farmers designate it, which seems possible because you paid in advance (you didn't buy on credit), then you'd have a problem. And because it's a farm share with the uncertainty that provides, I don't know if we can look to cases of ordering specific goods for precedent. That is, ordering specific chameitz from Amazon might be a different case, because you made a specific order and because if Amazon fails to deliver it you can request a replacement. With a farm share you're taking your chances; maybe you get a big pile of wheat berries, or maybe the crop had problems and you don't.
When does a subscriber to a seasonal farm share become the owner of the items in one of the deliveries? What would a customer need to do to avoid the problem of owning chameitz if a share includes any?
(I'm willing to split off that second question if people think that would be better; I don't know how complicated my question is.)