Compensating for e-mail reply

[Contestboy]

At our call center we have a helpdesk made up of our senior reps that fields the majority of our reps questions. They are compensated with an increase in their hourly wage for the times they're on the helpdesk. Recently we added a new reponsibility that has them replying to customers e-mails and give them the same compensation.

Sooner or later we plan to route e-mails to our reps (One moment they would get a call and the next it might be an e-mail that pops up on their screen) and need to come up with a way to compensate our reps. I'm looking for opinions on how to do this while maintaining the quality of a reply within a timely fashion. Any suggestion is appreciated.