A buyer needs an invoice, where do they find it?

Short answer: They get it themselves on the shop: once signed in, the menu under their name leads to My orders, and every order there carries its invoice, which can be read on screen and downloaded as a PDF. There is nothing for you to do.

What to tell them

Go to the shop and sign in with the email address used for the order.
Click their name at the top of the page, then My orders.
Open the order concerned: the invoice is shown there, with a link to download it as a PDF.

That same place lets them follow how far along their products are.

The invoice is issued by DAGOBA, who takes the payment for the order and delivers the products. Your organisation has no invoice to issue for a purchase on the shop.

Going further

What do the amount lines on an order mean — the same amounts, seen from your side.
Can I change, cancel or refund an order