Updating Reccuring Invoices Based On Client Tier

We are currently having to manually update our recurring invoice line items when clients move up a price tier.

Some Background: We run a SaaS company where billing is based on the number of products our system processes. When I client add’s more than their allocated bracket (e.g. original bracket was 1 - 500 products @ R250 p/m to 501 - 1000 products @ R500 p/m). If we don’t manually capture this change, we still bill them for the lesser tier meaning we are losing money. Is there a way to push new line items to update the recurring invoices.

We understand we would first need to gather this information prior to us pushing it to Freeagent.

Any help on this issue would be greatly appreciated.