EPID issue for AddFixedPriceItem

Question

I'm trying to use AddFixedPriceItem to add listings to my eBay store. I'm including ProductListingDetails as follows:

{
    IncludeeBayProductDetails = false,
    IncludeeBayProductDetailsSpecified = true,
    UseFirstProduct = true,
    UseFirstProductSpecified = true,
    ProductReferenceID = info.eBayProductId,
}

I'm using the eBay.Service NuGet package to make my call. However, when the item gets listed, it doesn't include the EPID or any reference to the product. What am I missing? I've left IncludeeBayProductDetails as false, as I don't want to overwrite my own title/description/images/etc. What else do I need to do in order to get eBay to pick up this added information?