You can facilitate this by invoicing the client yourself. Burst SMS does not have the facility to charge any kind of recurring monthly fee. Your client can still login and send their own messages though via our I Pay accounts. When you ADD CLIENT, under Pricing select I Pay, you can then either choose to allow the client to bill their activity directly to your account, or you can allocate them a set number of SMS credits.
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 then 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 then 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 un-used credits just 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.
You will quickly see that Burst SMS offers a very flexible array of methods for you to tailor and create packages in whatever way you see fit.