"Can I cancel the order if I don’t willing to fulfill it?"

You can cancel but you will get a BIG ding from Amazon when you cancel expedited items as well as risking bad feedback from the buyer. If I have to cancel an order I make sure I send a message to the buyer letting them know about it BEFORE I cancel. If you wait until after the email messages will probably be locked down with no way to message them!

The ONLY time I have had to cancel orders is if there is an inventory error OR the buyer is being 'difficult' and it looks and sounds like a scam attempt!

I would NEVER cancel an order due to a problem with the cost of shipping. THAT is a mistake on my (your) part for not charging enough at the time of the sale!

"The “Buy shipping “ is way too high, even the buyer already paid a portion of shipping fee."

WHY are they paying only a PART of the extra cost? Change your shipping settings so that you collect enough money to cover the costs.

"Is there any other workaround? "

See what the costs are on other sites such as Stamps/Endicia/Dazzle or Pirate Ship. They can be less ESPECIALLY if you are using UPS than what Amazon charges!

Ship the order, take the loss, and then FIX YOUR SHIPPING TEMPLATE.