Wholesale and invoicing customers

[HHampshire]

Hi, 

We have set up a new Shopify store that sells wholesale products only. The wholesale customers have an option to pay immediately or request an invoice to be sent so they can pay within 30 days. This payment would then be taken via Bank Transfer or CC separate to Shopify. My query is, what will Shopify charge us for these transactions? Everything but the transaction fee or will a fee be added for a general transaction fee? Many thanks!