i was just wondering suppose i was using DC (from here) and went to the post office and mailed

the book but forgot to tell PBS I mailed the book would i still get credited when the post office scans the book?

I've almost forgotten to mark books I've mailed as well and I'm sure I will one of these days. But I don't think you would get the credit as the system wouldn't have known it was mailed and assumed it wasn't. What you can try to do is contact the member you sent the book and let he or she know it is coming so they can put a wishlist on hold or not reorder the book. He or she might have requested a second copy already so you'd be out the credit but if they haven't yet they can still give you the credit from the transaction archive.

No, you don't automatically get a credit unless you have clicked the Book Has Been Mailed button by the deadline.