Selling food online nationally (US based)
The threshold for needing proper nutritional labeling in the US is something like $500k and 100k units (not sure the exact level). If you're selling in low enough volume, you essentially fly under the radar, though certain retailers may refuse to carry your product.
If you're going to meet labeling requirements, there are no shortcuts--you need to have proper nutritional analysis done and follow all the rules for ensuring it's accurate. Using a co-packer to manufacture and package your product comes with the benefit of their expertise in this sort of labeling and quality control.
If you're going to be doing any significant number of sales, or generating a significant amount of revenue, you probably want to look into having proper business insurance and finding an attorney to help you navigate legal requirements related to your venture. But that's far beyond food &amp; cooking, and gets to be off topic for this forum.