shippingServiceCost not being returned despite buyerPostalCode being included (findItemsAdvanced)

I need to pull shipping costs so I can display accurate pricings on my site. To do this, these docs say I need to include buyerPostalCode (if the shipping type uses rate tables). Despite including this, I am still not getting ShippingServiceCost returned. Does anyone know how to fix this?

For reference, my headers and parameters look like so:

api = "https://svcs.ebay.com/services/search/FindingService/v1"
headers = {
"X-EBAY-SOA-SECURITY-APPNAME": "(our api key)",
"X-EBAY-SOA-RESPONSE-DATA-FORMAT": "JSON", "X-EBAY-SOA-OPERATION-NAME": "findItemsAdvanced"
}
parameters = {
"keywords": search,
"sortOrder": "BestMatch",
"itemFilter(0).name": "AvailableTo",
"itemFilter(0).value": ship_to (variable we have for shipping, often "CA"),
"itemFilter(1).name": "Condition",
"itemFilter(1).value": 1000,
"aspectFilter(0).aspectName": "US Shoe Size",
"aspectFilter(0).aspectValueName": 8.5,
"paginationInput.entriesPerPage": 1,
"buyerPostalCode": "M5G2C9",
}