While I'm not in the screenprinting business, I do Accounts Payable work with my current sidejob, and I can tell you from an accountant's perspective: the more information the better. People tend to forget exactly what they had ordered at what price because, let's face it, we're all busy.
What I reccommend doing is keep it detailed, but keep it clean. Us account payables folk like having information at our fingertips, but the last thing we want to do is read an encyclopedia. So, at the very least, here's what I suggest including in your invoices:
- Invoice (or Reference) number
- Date
- Job(s) (By Breakdown)
- How Many Shirts
- How Many Colors (Per Design)
- Specialized Charges (such as specialized print locations, additional work fees, applicable credits, etc.)
- Payment Terms
- Additional Information (if you feel that there is something you could provide to answer a question someone may potentially have, answer it!)
Those are the basics, of course there are other things you can do as well, such as coloring your invoices differently (with hundreds of invoices per week, that actually helps our job out A LOT.) If there is one thing that I absolutely suggest you provide on your invoice, it would be to CLEARLY PROVIDE A DIRECT CONTACT PERSON AND PHONE NUMBER. That way, if any one does encounter some questions, your account rep could answer them right away and clear any confusion in a matter of seconds.