Seller cancelled the trade without informing me and I have paid by Afterpay

I bought a door mat from a seller in Auckland. I won the listing and made immediate payment by Afterpay. All of a sudden this seller cancelled the trade and refunded to Afterpay. When I asked why, the reply was NZ Post too expensive to post its $17.99 and the doormat was $5.99. It was stated clearly he used NZ Post and I have paid without any complaint. Does this seller has the right to cancel the trade without letting me know? I told this seller I dont mind NZ Post so long as it is not Aramex. Too expensive isnt his problem and I dont mind and paid for it. I suspect he is linked to another seller from the same suburb who made enemy with me 2 years ago when Aramex lost the parcel and that female seller gave me a red face and refused refund. In the end TM refunded me. Maybe they are partners in business and that seller told this seller not to deal with me. That is the only reason I can think of. Secondly why are some TM listings won and reference starts with KP instead of P? It is not visible on mobile mode after I have won the trade and it doesnt allows feedback. It is only visible on Desktop mode. Why did TM changed the reference to KP instead of P? Some sellers find it annoying and they dont like it. Please advise. Thank you.