Braintree setting amount and avoiding rounding errors

I am creating a web app using PHP/Laravel and Braintree for payments. I am using the Braintree dropin.

A simplified flow of the web app:

1. User uploads a file and selects a service from dropdown, submits form
2. User is shown quote/price which is calculated by taking the uploaded file's word count * selected service
3. User selects payment via Braintree drop-in, submits form to pay
4. Web app fires Braintree_Transaction::sale()
5. User is shown success page

Charging the customer

Regarding the above, I would like to get some advice on best practice for setting the amount in the Braintree_Transaction::sale() method call in step 4.

In the Braintree PHP example on github the amount used in the Braintree_Transaction::sale() method call is taken from a text input in the form, this can be edited by the user, I assume for example purposes.

In my web app flow described above, after step 1, would I be correct to set the quote/price of the selected service in the session then show this value from the session to the user in step 2 and finally pass the value from the session as the amount to Braintree_Transaction::sale()? Or is there a different recommended approach to take? My main concern is charging the customer the correct amount.

Avoiding rounding errors

As the quote/price is calculated in PHP by taking the uploaded file's word count and multiplying it with the service a customer has selected this will at times involve floating point numbers. E.g. Word count is 1000, selected service is proof-reading which is x1.5, so quote would be 1500 => £15.00.

If my memory serves me correctly Stripe represents, for example, one pound (£) in pennies 100. This is nice as it helps avoid rounding errors. Braintree on the other hand I believe expects one pound (£) to be represented with the decimal point 1.00.

What would be my best approach to avoid any rounding errors in such a scenario?

Thanks for any suggestions and please ask for more details if required I've tried to make this as brief and general as possible.