Using connected accounts on Stripe, and using javascript,
I need to show user the total amount that he will pay for an invoice from a net amount.
For example, user has an invoice of 1500$ to pay, and I need to include Stripe fee and the application fee (my fee), and I need to show user that he will be charged 1500$ plus 68.68$ in fees which makes it a total of 1568.68$. I need this amount to match exactly what Stripe will charge to process the payment.

For that, I currently have this small script:

    const stripeCommissionRate = 0.029;
    const stripeFixedFee = 0.30;
    const myFeeRate = 0.0146;

    function calculateFees(netAmount) {
      // Calculate gross amount to do calculations (reverse engeneering from Stripe's math) 
      let grossAmount = (netAmount + stripeFixedFee) / (1 - stripeCommissionRate - myFeeRate);

      // Calculate individual fees
      let stripeFee = stripeFixedFee + (grossAmount * stripeCommissionRate);
      let myFee = grossAmount * myFeeRate;

      // Round individual fees
      stripeFee = parseFloat(stripeFee.toFixed(2));
      myFee = parseFloat(myFee.toFixed(2));

      // Calculate total fees
      let totalFees = stripeFee + myFee;

      // Calculate final gross amount from rounded amounts
      grossAmount = netAmount + stripeFee + myFee;

      return {
        grossAmount: grossAmount,
        stripeFee: stripeFee,
        myFee: myFee,
        totalFees: totalFees,
      };
    }

However, this is not matching all the time. There’s got to be some special roundings or another way to do the math and that is the help I’m hoping to find here.

Examples:

Working example 1

For a net amount of 1500.00$, that returns
grossAmount: 1568.69
stripeFee: 45.79,
myAppFee: 22.90,
totalFees: 68.69

Working example 2

For a net amount of 1000.00$, that returns
grossAmount: 1045.90
stripeFee: 30.63,
myAppFee: 15.27,
totalFees: 45.90

Failing example 1

For a net amount of 49.99$, that returns
grossAmount: 52.58 <– on Stripe is 52.59
stripeFee: 1.83,
myAppFee: 0.77,
totalFees: 2.59 <– on Stripe is 2.60

Failing example 2

For a net amount of 299.98$, that returns
grossAmount: 313.97 <– on Stripe is 313.98
stripeFee: 9.41, <– on Stripe is 9.40
myAppFee: 4.58,
totalFees: 13.99 <– on Stripe is 13.98

Whenever I try to adjust something to fix one failing example, it ends up breaking something that is now working.