A couple of options and a couple of hurdles.
1. Probably the easiest way is to create a custom listing for all the items the buyer wants with total price and
the calculated shipping and coordinate with the buyer on the timing of the posting so they can purchase
it as soon as it is available. Use BIN/IPR for the listing.
2. Buyer can attempt to place items in the shopping cart and once everything has been selected they
can complete the checkout process. The cart has been known to have some issues with combined
shipping if your items are all set to BIN/IPR and there may be issues if you are accepting best offers
on the items and the buyer has to provide a payment method before they make an offer. Once you
agree to the offer it is an automatic instant payment.
3. Buyer can purchase items individually and when you go to ship them you can simply use the same
tracking information on each of the orders, pack everything into one box and issue a shipping discount
refund to the buyer for any excess postage they may have paid.
