Recently I have had 2 orders cancelled by the sender, but when I look at my transaction archive, it doesn't say the sender cancelled, only "cancelled". And both swaps still say "have you received this book" even though neither of them was ever mailed. Does it matter to TPTB which end of the swap does a cancellation?  Or does that not even get considered in looking at determining if an account might be suspended?

If you let your mouse hover over the red X on the left side of the listing in your transaction archive, it will say who canceled the transacton.  The "have you received this book" will always be there -- it's part of the furniture.

I think book cancellations very rarely cause accounts to be suspended.  Probably only if someone cancels a whole bunch in a short period and even then they contact the person.  THey wouldn't close someone down because a family emergency cause them to cancel a bunch of pending transactions.

So I wouldn't worry about it, especially since you are on the other hand.  I've had to cancel several transactions over the years as a sender and it did no harm to my account.

Does it matter to TPTB which end of the swap does a cancellation?  

yes, of course.

The reason that you still have a "has this book been received" button is so that in the case where someone mails the book out but forgets to mark the book mailed, thus "cancelling" the transaction, you could still mark it received from the transaction archive.

I really think it woudl take a large number of cancellations, or perhaps a complaint from someone else reporting some kind of misbehavior, for PBS to even go look into someones account just because of cancellations.

If you cancel due to noticing a problem such as highlighting or due to issues at home such as health problems, this is treated very differently than if you decided that a book was too heavy to mail.

It sounds like the cancelations you have experienced might be to folks failing to mark a book as mailed. Not only does PBS count how often this happens, but the sender also risks not receiving their credit.

Books cancelling for not being marked mailed are probably quicker to get an account flagged then ones where the sender manually went in and cancelled even if they didn't give a reason.