I just bill them when I can cope with doing it. Ha!
I have a WordPerfect document that I treat as a template, then save as PDF, which is my clients' preferred format. The information is more important than the layout, and I'm consistent with each invoice, so they know what to expect.
I did a temp job many years ago, and consistency of format for each vendor was more important than prettiness. I quickly learned to where to look for information from each vendor, since all their invoices were different.
My bigger issue used tp be tracking my invoices, but I refuse to buy a piece of invoicing software for that. I finally created a table in a document that I update at certain "trigger" times: when a check arrives, when I invoice, or when my checklist reminder comes up. That way, nothing gets left behind.