So that's basically what I was thinking with minimum prices per shipping option. If your order is <$1, you can't do tracking, but you can still do plain white envelope, for example.
I think sebi's point still stands there though because some sellers could literally just not offer a shipping option with no minimum.
This is probably dangerous, but maybe create a shipping option that's always there for everyone and can't be disabled, which would be up to 6 cards in a PWE with no minimum, priced at like 50 cents (current cost of a stamp plus a few pennies for an envelope)? I know it's getting into gray area with creating something that sellers have to offer, but it also seems like a reasonable option to have anyway. Obviously this only works in the US, maybe Canada (not sure what their post is like), but that's probably the majority of buyers and sellers anyway.
Alternatively you could just encourage people to offer that option without requiring it. Sure, some sellers will just set a minimum order value on everything. The thing is, some sellers already do that anyway. They just write in their profile that they don't do orders under $5, for example, and then just cancel and refund anything else. It's just more of a headache for everyone. Or, as I mentioned, they'll just set a higher shipping cost, since that can't be avoided, and then apply discounts.
Ultimately I think that sellers that run into this problem frequently enough will find a way around it anyway, but it would be nice to have an officially supported way that reduces the headache for everyone involved.