First let me start by saying this is something handled by your accounts payable department (hopefully you have one or some department that handles or helps out in the process).

Typically this is the procedure to order something:

Opportunity–>Quote–>Purchase Order #–>Sales Order #–> Invoice

1. VAR > Customer = Quotes
2. Customer > VAR = PO
Not Contract Until:
A. VAR accepts terms by signing PO
B. Acknowledgment and Acceptance of Order – it becomes legally binding contract
3. VAR > Customer = SO :OPTIONAL
Customer may request SO to view exact details about product, price, terms, & delivery
4. VAR > Customer = Invoice
Once everything has been completed, an Invoice is sent out.

The invoice is something that the Vendor sends to the customer after the order has been completed. So shortly after delivery of the product when buying goods or after a service has been completed if it’s service-orientated. Accounts Payable will go through some checks before the invoice is paid. Some of those checks are prices, quantities, and terms to make sure the vendor fulfilled their PO legally binding contract. Plus, the VAR can not release the Order until a PO is signed.

I’ll break it down even simpler:
1. (Me Customer) Send me a quote
2. (VAR) Sends a quote
3. (Me Customer) AP creates PO
4. (VAR) Accepts PO, ships products or does service
5. (VAR) Sends Invoice
6. (Me Customer) AP Verifies and does key checks, pays invoice