A buyer purchased an item from me and before I sent the invoice, she indicated she was interested in another similar item and asked about combined postage.  I am happy to do so and messaged her back. Within a short time, this second item was purchased -- by another buyer -- but I didn't realise it was a different person and have really mucked up the invoice of the first buyer. The Help page infers that I can send a revised invoice, but I haven't been able to find that option anywhere. Is it possible?