WebSMS's Whitelabel solution provides functionality for resellers to run their own SMS Aggregation plugging into SMS Gateways like Clickatell, Panacea Mobile, InfoBip, SMS Relay, BurstSMS, Nexmo, End2End Mobile, etc. using HTTP or SMPP interfaces.
Price on message costing vs credit basis to give customers accurate pricing. Make markup from margin on your account. Specifiy which SMS Gateway to utilise to submit Mobile Terminated traffic.