If the documentation produced by the store by default is missing something that you need then let me know and I'll try and produce what you need.  After your item ships you will be provided with a direct download link for an invoice as a PDF file, which is usually sufficient for most purposes.
If you need some documentation before you order, then the quickest solution is to create an account in the store - complete with your company details, VAT number, billing address, shipping address etc - and then add what ever items you want to your cart and begin the checkout process.  Once you get to the point where you are asked for payment details email me back (or use the 'contact me' link at the bottom of most pages - but probably not the checkout page for security reasons) to let me know that you would like an invoice, and I can use the data you entered to product an invoice for your approval/payment.  That invoice should pick up all the data associated with the account/order that you created so if you included all the details that your company or accountant requires then that should be added to the document correctly.