If you would like to allow your customers to pay by invoice, you'll find an option to enable this on your Online Store Settings. Just navigate to Market, select Online Store, and check the box to "Allow Pay Later (Invoice)".
This lets your customers make an online order through the store, but not supply a payment method at the time of ordering. You'll be notified about the order, and you'll see it shown as Payment Due with a status of Ordered. You can then review their order in Farmbrite, make any adjustments to it, and then send them an invoice for a final amount due that they can pay.
Tip: You might also want to enable the setting for "Require Email at Checkout" if you plan on emailing these invoices to your customers.
This is particularly beneficial for frequent customers who you might invoice monthly on a "net 30", as they can request products and submit orders through the online store without having to pay at the time of ordering.
This could also be useful if you sell packages of meat that can vary in weight, since you can update the quantity before invoicing the customer to reflect the exact package weight and price.
In our example below, the customer is purchasing 15 pounds of Bison Filets. These cuts can vary, and it would be tough for us to give the customer exactly 15 pounds. Notice that we encourage the customer to pay by invoice in the item description in the shop, letting them know we will send them a finalized invoice later. After checking our stock, we can fill this order with cuts that total to 15.36 pounds, so we remove the 15 the customer requested and replace it with the actual amount provided. We then save, send the invoice, and the custom pays!
You can see how this might also serve as a "reservation" order, where customers request am amount, but pay later when the amount is finalized. As long as you communicate with your customers and set expectations correctly, they'll be able to reserve the amount they want online, and then pay when the item is ready. This might work great for shares of an animal to be butchered at a later date; when the final weight is tallied your can add it to the invoice and have them pay for what your provide.
Let us know if you have any additional questions on how this might work for you.