These are some general tips. Different sellers have different approach to postage.
Some drop correctly stamped envelopes into mailboxes, some have 'pre-issued' sticker-tags, some have post office connections, some have their staff going around and posting. Some all above and more.
Depending on the volumes one sends, i believe, at the end it all comes to just being organised/professional.
Also, there are different methods of delivery depending on the country.