I sat down with a spreadsheet and figured it out back in December 2017. If my math was correct, the point at which the 5% they said would cover payment processing actually did was for pledges of $14.29 and above.
"The Fourteen Twenty-Nine Hypothesis"
https://siderea.dreamwidth.org/1371510.html
This meant that if they weren't getting a special deal on payment processing, if they had tried to keep to 5%, they would have been losing money on any pledge less than $14.29.
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