Especially because we cannot make items in a bill "billable to a customer" right now, and because we need to attach those bills to an invoice so that the customer can see what they're billed for, it's very inconvenient not to be able to keep attaching files as we add more lines in an invoice showing the expenses incurred due to bills from our vendors. This will not be that crucial once you make it possible to make items withing a bill "billable to a customer" as in an expense entry.