Collect items to bill and credit limit for clients
I suggest to make the option to "collect" items to bill during a set period of time or to a "credit limit" for the client before generating an invoice.
Lets say that client A will get invoiced only on the 15th each month for all services of that month.
Or that client B has a "credit limit" of US$200, and that no invoice will be generated before the "credit limit" is passed.
This will avoid a lot of small invoices and both we and our clients will be happy to save some money for accounting.