Let me start by saying - it happens and I am not mad at the trade partner here - this happens and I am fortunate enough that it isn't a huge deal, but it is worth sharing in case it helps others.
Declared value when filling out customs information can have an impact. In this case, the post office in the US, not even the trade partner, put $100 value to the package when sending. The trade partner is looking into it, but I suspect they were asked about insuring the value "up to $100" and that is where it came from.
Well, there may not be duties - well, at the time of this trade anyway - on values under $150, but there is sales tax on values over $40.
So when this arrived, it cost $28 CDN to accept. Almost half of that being the customs "fee" for them handling, charging and collecting the sales tax.
A good reminder to keep an eye on how customs forms are completed, as I don't think these are worth $40 let alone $100. The trade partner was going to go back to the post office on it and let me know what happened, and if they can at least get a refund as they even paid $17 USD shipping....yikes.