Simple is the way forward, less paperwork the better, have enough to cover the work done without having a mountain of paperwork that you then need to store for 5 years!!
Why have an invoice pad and a report pad for the same job? A report pad is for contract customers who get a separate invoice sent by email or post and an invoice pad is for one off jobs. The invoice pad only needs enough space to have the customer address details, your details and the job description that's been carried out along with the relevant costs.