Sometimes customers will place multiple
orders. It is convenient to them (and to you!) to combine
these into a single order. This is particularly likely
to happen if they are buying on eBay.
If you see you have multiple orders from the same customer, just
use the 'search' function in Orders and Billings to locate orders
for a single customer (search by email address or name).
You will see a display that has all the orders for the email
address. Click the 'combine' check box of the items you
want to combine and click 'Combine' at the bottom of the page.
All the line items on each order will be combined into the first
order you've selected. If you use automated shipping or tax
calculation, those will be recalculated automatically for the order
into which the items have been combined.