There's currently no Google API for uploading feeds of products to the Products feature. The Google My Business Management Service includes optimization of the new Products feature in GMB only if it is available on the listing. This feature is still in Beta testing and sometimes products or product photos are removed due to glitches with the Beta version.
We’re able to upload up to 20 featured products initially and can add 5 new products each month. For this service, we currently add products by having clients complete a spreadsheet of their products with the information required.