With regard to postage I usually give a posted price and then send by the most economical method
Just sold a small item for 15 quid, standard parcels is £4.41 and 1st class is £5.90, can't see anything wrong with using the cheaper albeit slower method.
Probably going to get a few people disagree with this next view, but if I had to decide who to sell to it would definitely sway me if one responder was abrupt and impolite with the other friendly with please and thank you
Manners matter in everyday life and that carries over to online with me too