A fixed fee per transaction takes a larger share of revenue when the order is small. Work it out both in money and as a percentage of each sale.
An example with numbers
A hypothetical fixed fee of $0.50 is 5% of a $10 order and only 1% of a $50 one. Any percentage platform and payment fees come on top.
Try it with your numbers
Try both prices with the same fixed fee. Use the rates in your own contract and check what each percentage is calculated on.
Example figures for learning purposes. They are not a recommendation and do not replace professional advice.