Shipping cost changes after checking out?

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I’ve had a couple of customers with the same problem now. The first was ordering from France and said they saw shipping was only $8 until after they checked out it was $22 and they wanted to cancel their order because of the shipping cost..
Shipping from the us, it’s obvious the true cost would be $22, but I’m wondering why they would have seen $8 shipping if it weren’t the amount automatically calculated by usps..
Another customer just today said when choosing their shipping method it was listed as $3, but when checking out (they use Apple Pay) it increased to $8, which again, seems more realistic. I have all of my shipping rates to automatically calculate based on weight using my standard box, so I’m curious if anyone knows why it’s showing such a low number before increasing to the calculated amount?!