Hi Jenny - When you offer the option for a buyer to purchase more than one item, you must also specify a combined shipping policy. Then the buyer knows what the additional shipping costs will be, if any, for more than one item.
To do this just expand the 'Shipping Tools' section on the listing page, scroll down to 'Combined Shipping Discounts' and check the box next to 'Apply my eBay promotional shipping rule'. This refers to your shipping discount settings in your ebay preferences.
If you click the info icon (question mark) you will see this message:
Follow these steps to get to your eBay Preferences page:
1. Log into your eBay account
2. If you are not on your 'My eBay' page, click on 'My eBay'
3. Click on 'Preferences' located on the left side-menu under the 'My Account' section
4. Locate the 'Selling Preferences' section
5. Click on 'Show' to the right of 'Shipping and Discounts'
6. Then click 'Edit' to specify what you want your preferences to be
On the other hand, if you are selling your two items as a 'lot', so that the buyer can only purchase both items together, then you need to specify them as a LOT, rather than as INDIVIDUAL ITEMS. Then the error message and above requirement will no longer apply.