Seems about right, the only thing is that instead of worrying about one charge back
now you can have multiple.
Reading a bit
about this with Uber you cannot split a bill afterwards, and there is always the 'primary' person who will get charged if the other payments fail.
No experience with Braintree but stripe charges 2.9% + $0.30 so I would do
((the amount + processing fee) / by number of people) + ($0.30 * number of people)
Depending on your payment provider there can be a difference in cost between processing visa and amex cards ?
Things can get *really* tricky if in EU bill is split over multiple people from other countries (other VAT rates) or US multiple states and tax is different per person ?
If possible would avoid that by using the primary person's details for amount calculation.
Another thing to keep in mind is that you will have to provide everyone with receipts and you cannot store all cards in a single account. This is probably the main reason Uber did this as it is a nice way to growth hack since all users need to create an account first in order to participate in the bill.