I've had UPS outright lose partial shipments on my twice now - once coming from the US and once within Canada. I've also received deliveries from them that have been damaged within shipping but our driver just leaves it inside the lobby of our building and runs away like a coward rather than own up to it. On more than a few occasions, UPS has charged absolutely exorbitant "brokerage fees" on cross-border shipments that would not otherwise incur duties. In my experience, these fees have ranged from 50%-100% of the cost of the item ordered and they hold your shit hostage until you pay it. So when I ordered somebody a $60 art book direct from the artist as a birthday gift some years ago and had to pay $60 to liberate it from UPS (let alone make my way out to an extremely inconvenient location to do it suddenly went from a birthday gift I could just barely afford to get them to one it took a while to recover from. To add insult to insult, one of the drivers in our area has been incredibly rude to my wife on two occasions now. And, of course, calling in to register a complaint or try and locate the fifth box from a partial delivery is just an absolutely convoluted mess of robots, recordings and unhelpful people and options.
Truly the absolute worst of it for me though has been the bullshit "brokerage fees" which is obviously a strictly cross-border thing. FedEx and DHL also pull the same bullshit (I've also had DHL charge me an $11 fee to collect 50 cents worth of tax that had gone unpaid by a shipper before). This particular issue obviously wouldn't be a factor in your domestic shipments - and, ultimately, I understand that you as a business have to do what's best for you to make sure your asses are covered and it sounds like USPS has fucked you over far worse than UPS has fucked me over so wanting to move away from that makes perfect sense.
To be fair, I don't recall being able to order to Canada through your website anyhow - the export restrictions you mention make that make a bit more sense, so really my disdain is kinda moot on this point. I just get triggered by UPS and have an exceptional hate-on for them given my experiences. I will avoid purchasing from places where UPS is the only shipping option - especially if it's crossing a border.
All that said, if moving your shipping strictly to them is going to be better for Plaid Room that's the most important factor. And those shipments being all, or mostly, domestic eliminate the most grievous bit of bullshit from UPS's arsenal in their dealings with you - so more power to you. Despite being unable to directly support y'all due to geographic factors, I really love and appreciate the shit out of what you do and how well you do it. As such I wish you nothing but success and a process that suck less!