
Is it a calculation error or am I missing something? A popular company like Plimus that handles thousands of payments daily can't make this mistake, I thought.

Is it a calculation error or am I missing something? A popular company like Plimus that handles thousands of payments daily can't make this mistake, I thought.
Yes, it's a calculation error, or, more likely, a typo; if you change the number of orders to 100, then everything works out.
What worries me is that on a \$49.90 transaction, there's a commission of \$4.99, while on a \$50 transaction, the commission drops to \$4.50. So, if the transaction is \$49.90, it pays you to lie and report it as \$50, and come out of the deal 49 cents better. Similarly at all the other percentage breakpoints.