I buy a lot of soap from a lot of different people & 1 -3 bars generally come in a padded envelope. Boxes do add quite a bit of weight to an order & you have to up the price of shipping for the extra box weight.
1st class mail is handled the same & is as quick as priority mail, so anything under 12oz (I think) I would suggest shipping in an envelope 1st class to save your customer money. They will thank you for it!