Pinterest accepted my application to sell directly on their site, but my first "ingestion" failed, as, unlike Facebook, they require google_product_category to not be null. Is there a way to fill in this value? They also won't accept manual uploads of the product CSV file.
I also have thought about putting the CSV file elsewhere on my website, but I don't know if I can do that.
These would be the two categories I would use:
9 - Home & Garden > Decor > Artwork
500044 - Home & Garden > Decor > Artwork > Posters, Prints & Visual Artwork