UPS is the problem regarding high shipping costs.
UPS is the Ticketmaster of couriers.
When I first started buying gear on eBay, I bought something that was within the province. The seller shipped the item with UPS. When the item arrived, I had a little chat with the courier and we go onto the topic of buying guitar gear on eBay, etc. He told me, 'whatever you do, don't use UPS across the border!'
Heeding his warning, anytime I bought something on eBay, Reverb, guitar shop sites, etc., I would always use USPS/Canada Post. In the hundreds of transactions I've made over the years, I never had any issues with USPS/Canada Post.
However, one time, I bought something from an eBay seller in San. Fran. He shipped the item via USPS/Canada Post, as requested by me. The item made it all the way to the local sorting centre, but for some reason, possibly a damaged label or missing information, the item was sent back to the seller. The seller felt bad, so to expedite the shipping, he used UPS and paid for the shipping. He told me after the fact, so it was too late for me to request USPS/Canada Post again. I think I paid around $50 for the item. When UPS showed up at my door, the driver was requesting $55 in brokerage fees before he would release the item to me.
If I recall, the only time I specifically used UPS, was when I bought guitar, an 87 LPC. It was just before Christmas and I was hoping to get it before Christmas, so I thought UPS was the best option. In the end, it arrived at the local terminal too late and it sat in a cold truck over the holiday's. Fortunately, when I did receive the guitar, all was good, but I did have to pay quite a bit in brokerage fees, which were on top of the shipping fees I had already paid upfront.
Several years ago, I received a letter in the mail, from a law firm. I opened it up and there were two cheques, made out to me. They were reimbursement cheques for those two UPS shipments, due to a successful class action lawsuit, that others had filed against UPS, for charging outrages cross border brokerage fees.
Here's the Notice of Settlement;
https://collectiva.ca/wp-content/uploads/2019/02/ups-NOTICE_OF_SETTLEMENT_APPROVAL.pdf
I always use USPS/Canada Post and never had a problem, except for that one time. I just recently bought an LED sign from an eBay seller in California and asked if he would ship via USPS Expedited Parcel, which he did. I ended up paying about $150 CDN, of that about $50 was shipping. It was delivered to my door in 6 business days, with no additional charges. From my experience, customs will typically not charge duty on items less than about $200. Also, the brokerage fees are included in the shipping cost, so no surprises.
Anyway...those are some of my experiences from nearly 20 years of buying and selling gear internationally.