It is a variation from contract and therefore subject to negotiation by both parties. Ie They ask if ok, if you want post refund it is up to you to ask before delivery and it is then ok for them to say I will post it then. There is no right or wrong, only negotiated alternative.

TBH for me as a seller my time and effort is worth more than cost of postage. So it would not be done for financial gain, only to provide either a quicker or safer delivery. Likewise pick ups for me are more inconvenience than post cost, and only allow for items where there is no alternative