So usually when I sell multiple items to a buyer I send them an invoice and adjust the postage accordingly - there is no such option there at the moment - there is a link to combine items so I click on that and then try and collate them there and get this message
Sending invoices is temporarily unavailable
We're working to fix this issue as soon as possible. In the meantime, if you want to send combined postage discounts, we recommend refunding the postage difference to your buyer after purchase.
