I use iZettle too, mainly for Craft & Design markets, and soon in my wee pottery shop in my new studio.
I have my 'customers' set up by type of sale, since I don't have many recurring individual customers, so my client list is 'craft markets', 'open studio sales' ,'gallery sales', and then accounts for individual reseller outlets as needed (since they will have recurring).
I mostly use iZettle for the first two - markets and studio sales, recording every sale whether cash or card and using it as a type of cash register, then I raise invoices in QF by date or by event (so, a weekend two-day market will be one invoice), and will be 'paid' by my iZettle bank (merchant) account.
In this instance, being able to import my izettle transactions for a particular date range to make up an invoice would be fantastic, as it would save me having to enter line by line on the invoice, but still allowing me to see what sold at that particular market/event.
Anyway, when/if this gets developed, I'll be very happy. If you want any help with reports or formats, etc, in the meantime, I'm happy to help.