You will get an email telling you when this has happened; the email will notify you that you have gotten credit.
See below for more about transit times. See If a book you sent hasn't been marked received yet... for more about this situation.
You will get an email telling you that Instant Credit has been granted; the email will notify you that you have gotten credit. Instant Credit is available to members with good swapping records - late mailing, non-mailing or an excessive rate of unreceived books will make an account ineligible for Instant Credit.
You will get an additional email telling you when the book has been marked received; if Instant Credit has been given, then no credit will be granted at the time the book is marked received.
Read more about PBS Printed Postage in FAQs about Printable Postage.
If you are sending a book to or from outside the contiguous US, it is a good idea to send a Personal Message to the requestor at the time you mark the book mailed, to let her/him know the package might take longer than usual in transit.
Related Links:
What happens after you submit a request for a book
What if the sender does not respond to my request within 4 days?
What if the sender agrees to send the book, but doesn't mark it mailed before the deadline?
What if I want to cancel a request I made?
What if the sender is taking too long to respond? Can I cancel and reorder from someone else?
How can I tell who's sending me a book? (or to whom I am sending a book?)
Box-O-Books Tutorial Step 4: Boxer Negotiation