Hi Anne,
Thanks for your reply. I'm not sure what "technology" would be required; I'm not an expert in e-commerce. I would think the issue could be solved via human interaction. For example: a customer requests orders be combined, so SDE cancels the existing orders and creates a new one with the same combined contents. Or, simply make notes on the orders that they should be held and combined when the final order is ready to dispatch, and any difference in postage refunded.
It's not quite the same, but I do place larger orders with Burning Shed (also in the UK) a few times per year. These orders typically include a number of preorders. It is their standard operating procedure to ship items as they become available, but a simple request to customer service and they will hold all items until all are available. From a customer-facing perspective, this is simply appended to the order as a text note. As they collect postage at time of purchase under the assumption that the standard dispatch policy will be followed, they will then issue a refund at the actual time of dispatch reflecting any difference in postage cost.
I would definitely recommend reaching out to Pete Morgan and the team at Burning Shed to lean on their 22 years of expertise as a small business shipping worldwide. I suspect they've dealt with many similar issues since 2001.