ImportAsset; POST /episerverapi/commerce/import/assets

This actually returns the task id (guid) for use in status request, so this wouldn't be the assetkey as this endpoint supports zip files, which could have more than one asset.

This is likely the issue when trying to do the asset linking and with it unable to find the media.

ItemAsset; POST /episerverapi/commerce/entries/{code}/assets

The {code} part is actually the entrycode or the "catalog entry code"/sku

Offical docs show this clearly:

var model = new ItemAsset() 
  {
    CatalogEntryCode = "Jackets-Blazers-Wrap",
    AssetKey = "fd761c1d-5692-48b6-b90a-1ddf4c7c5eb9",
    AssetType = "episerver.core.icontentimage",
    SortOrder = 45,
    GroupName = "large"
  };
var json = JsonConvert.SerializeObject(model);
var client = new HttpClient()
  {
    BaseAddress = new Uri(ConfigurationManager.AppSettings["integrationUrl"])
  };
client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token.AccessToken);		
var result = client.PostAsync("/episerverapi/commerce/entries/{entry code}/assets",
  new StringContent(json, Encoding.UTF8, "application/json")).Result.Content.ReadAsStringAsync().Result

Thanks for your input; I neglected to look further than the "swagger"-like overview on the following page: https://world.optimizely.com/documentation/class-libraries/rest-apis/service-api/#/ImportExport/ImportCommerceAssetsFile

I assumed the fact that I could include a binary meant that the binary was a single specific asset of which one could then link to an entry. I have since discovered that this endpoint has a much larger function than simply uploading a single asset: https://docs.developers.optimizely.com/customized-commerce/v1.3.0-service-api-developer-guide/docs/media-service

While I would argue that the function of the asset import endpoint diverts away from the RESTful approach of the rest of the API, the important conclusion here is to read the documentation more thouroughly 😃

/Stephen