We've noticed there doesn't appear to be an equivalent to AddItems with AddFixedPriceItem.We need to use AddFixedPriceItem and would be interested if there was a way to group individual items together like you can with AddItems.
There is no equivalent of AddItems for AddFixedPriceItem. However you can schedule multiple AddFixedPriceItem requests by sending them in a file and creating a job that executes the requests asynchronously. This is possible with the LMS API. AddItems defines one to five items and lists them on a specified eBay site. In contrast, when you use AddFixedPriceItem call with the LMS API you can list as many items that can be accomodated in the payload file which should be no greater than 15MB.