I've both sent and received playline dolls in a padded envelope, and never had any problems occur. in case there is enough padding between limbs and possible rub-prone surfaces (and in case the doll's hair is not heavily styled, as that will not be safe in an envelope) you should be fine
in fact, you might even attract more buyers with the cheaper postal costs the less packing material causes. I had several buyers actually request (inland) playline doll purchases to be sent in a padded envelope instead of in a box to save on shipping.