eBay no longer makes this easy!
The buyer can try adding the items to their shopping cart and requesting an invoice, but there are mutliple reasons that this may fail. We have been begging eBay to fix this for months.
As you say you can create a custom listing for them.
You can also set specific order size discounts and combined shipping discounts if you have a store.
You can also use the Reply with Offer and clearly list what items are included. Shipping gets dicey, because they buyer will be charged the shipping listed on the item the question was sent to you under. So you have to find out what the shipping amount the buyer is seeing and then add the difference to your offer price. (And then send a small refund or take a hit when eBay's shipping calcuator at checkout charges the buyer a different amount)
If a buyer purchases several items individually and they all ship USPS they can be combined when you print the label, but any refund or discount is up to you to do manually via paypal.
Member of the Grumpy Old Man crew