For example, if you want to sell a package for $100 per month for 1000 SMS and you don't want them to exceed that. Choose 'I will transfer SMS credits' as the Payment method. You will be able to Add Credits directly to that account from the clients screen.
When you Add Credits, your reseller balance will be charged for the cost. Your client can use these credits as they like until they run out. If they do not run out by the end of the month, if you choose to expire the unused credits, simply top them back up to the 1000 messages on whatever agreed billing date you have and invoice them directly again.
If you are not concerned about the number of messages your client uses, you can just let them send an unlimited number of SMS billing your reseller account directly. All SMS usage is broken down and displayed monthly per client.
Burst SMS offers a flexible array of methods for you to custom tailor your own packages in whatever way you see fit.