Issue with shipping cost

Hello, when I order books I usually buy 2 at the same time. Using USPS First Class Package International shipping, shipping is cheaper per book than ordering only 1 book at the time. I never order more than 2 books because USPS First Class Package International is no longer available and the next cheaper option is UPS and that mean getting screwed by UPS on fees and taxes when it come to Canada.

There seem to be an issue with the shipping cost, it doesn't make sense for 2 books to cost 133% more than 1 book with First Class Package International! (for the test, I used 2 copies of the same book "My Bride is the Demon Lord!?", but its the same for 2 books of the same price)

Using the shipping calculator and shipping to Montreal:
1 book:
First Class Package International at $15.20
UPS® Standard at $27.37
2 books:
UPS® Standard at $27.72
First Class Package International at $35.39

Usually, the shipping cost is $23.99 USD, that was back in June, and its has always been $23 for the last 3 years.

Damn, least yours is cheap.
for me it's;
2 books
First class - 50.11
ups expedited - 68.39
ups saver - 76.51

1 book
first class - 22.33
ups exp - 57.67
ups saver - 60.30

But yeah, first class is literally over double

delivery companies do change prices about ever year
plus it does go of weight and size of the parcel

from when i used to sell on ebay
if some of the item i used sell i was able to send in envelope which was cheaper as it cost me £1
but if they bought 2 or more it meant the size turned it into parcel which cost £3

so it was cheaper for me to shop them separately