I absolutely see why adding product directly in the API would be a desirable change. I'll move this over to our ideas section so that other terrific community members like you can vote on it and show interest for future implementation!
Yes! Add product via API is essential. There is a weird quirk with the shipstation api that does not allow an order to be updated with product information unless the product contained the information before the order was created. But the only way to create a product in shipstation (besides manually or using csv) is to let it be created by the order. @Moderators-Davis, would you kindly link to your idea thread? I would like to upvote so I can stop taking crazy pills.