Maybe all I know is there was surprise $30 on top of what I payed to WotC, just for one of the books to be slightly damaged. Not bad enough to return, but still pissed about it.
Never physical books from these guys again.
Honestly I deserve it for buying American in the first place.
tldr; Your extra $30 was not charged by WotC or Canadian border security and is not a US tariff; it was an admin fee charged by Purolator/UPS/whoever delivered your package. Any damage your package incurred is also 100% the responsibility of the courier.
Which courier was used if you don't mind me asking? Frankly, if your book was damaged during delivery, that would be on them, not WotC. If this had happened to me, I'd be taking it up with the courier.
Brokerage fees aren't always charged; it's a bit of a lottery. Typically, the courier only dings you this administrative fee if the CBSA (Canadian border security for any non-Canadians who might be reading this) deems your package worthy of their attention. This fee is applied by the courier, not the CBSA. Basically, if the courier has to do any extra paperwork for the CBSA, the courier charges you an admin fee. In the case of a book, which doesn't incur any sort of tariff or duty, your package would have been randomly selected for further inspection as part of the CBSA's security procedures, and the courier would have had to file additional paperwork with the CBSA as a result.
Edit: Unless the CBSA favours American shipments in its algorithm for selecting packages for further inspection (which is possible, who knows...), remember that any delivery from any country can trigger this inspection. That inspection in turn triggers the courier to charge you the corresponding administration fee. Just pointing that out as I noticed you partially blamed the fee on 'buying American'. I admire the patriotism though!