It's all negotiable so make sure you start a thread with your requests.
Prices for this service are totally negotiable (depends on the amount of service you order) and we typically like to hear your offer first so don't hesitate.
Payment can be done through donation system via PayPal here.
It depends on the customization, but maybe we will need access to your XenForo installation. Post credentials only if you are asked to.
Make sure you get us an account with admin permissions. Post your credentials in a new thread in this forum (it's private and visible only to you and customizeXF staff).