There are 3 ways to charge your clients:
1. They can add their credit card to your reseller account with which they can add credit to their account. You add a margin to SMS, numbers and/or keywords, this margin gets added to your reseller balance. They can top up variable amounts or you can set a package amount.
2. You can allocate a number of SMS credits to their account and then invoice your client off site. You can charge a monthly fee or whatever you want.
3. You can allow your client to send as many messages as they like billing directly to your account. They will not see any of your information. This is the primary way to resell API services.