Unlike post-purchase, an offer accepted on the thank-you page creates a genuinely separate transaction rather than modifying the order that was just placed — which is both why swap and auto-add aren't available here (there's no existing line item to swap or auto-modify) and why this context can support carousel multi-product layouts that post-purchase's SDK can't: it isn't bound by the same mid-authorization constraints.
This is also the surface UpsellPlus Earn uses for third-party partner offers, since it's the first point after an order is fully complete where showing an unrelated brand's offer doesn't touch the transaction that just happened.