Never mind, I found it. Although the specific term "Override shipping methods per transaction" doesn't show in the edit mode, but does appear in the view mode. Edit mode actually says: "Use the shipping fee in the transaction instead of my calculator's settings: yes, no." I selected Yes, and then in view mode it says "Override shipping methods per transaction" On. [If you've set up shipping preferences in your Profile and want to set a specific amount as the shipping basis for your button, you need to set "Override shipping methods per transaction" in your Profile to "Yes." To do it, go to Profile > My selling tools > Shipping Calculations.]
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