As the coins are now safely in the hands of a trusted friend, I will reveal all.
Firstly, an amelioration of my previous criticism: the staff at Lawrences are extremely helpful and have resolved the confusions caused by their systems cheerfully and without quibble. So the criticism goes to their admin system - AUCTIONET - and whoever in management decided this was a good approach to customer service, and made this an obligatory default for payment.
Specifically:
It insists on sending out separate invoices for each lot sold,
The process for requesting combined invoices is complex and requires manual intervention by AUCTIONET, not the auction house,
The postage costs are set very high, and by default are charged separately for each lot, with minimal discount for combined postage when achieved,
The payment process is buggy - it cannot cope with more than payment per session, so you have to log out and back in for each transaction,
By default it starts sending out abrupt "storage charge" emails after only 1 week and with no consideration as to the size of the lots.
So my advice would be: if you find out your auction house is using AUCTIONET, be prepared for frustration.