I am thinking about listing a large antique farm tool (seeder), but there's a problem

In Paypal, you can connect one box to another - the choice offered is "print another label for this order" or something like that. You can call them to clarify, they are very helpful.

UPS will ship bigger boxes than USPS; that might be an option.

Or, if the item is truly huge, you could list it as pick up only and refer people to Plycon or a similar company (move single items) in the listing.

Greyhound Buses also carry freight, by the way.