Add product to existing order

[gks]

HI,

 

Trying to do something fairly simple but can't find a way of doing it.

 

Example Client has a recurring monthly invoice with 4 items on it. Let say they have upgraded and want an extra of one of those items.

 

What i'm wanting to do it simply have the extra product begin when they are next invoiced, so it is simply added to the invoice.

 

Is this done through billable Items?

 

if so do i need to enter the due date and what is the invoice count? i want it to continue indefinately.