I received an email from a new startup that does flat rate pricing for their USPS labels. Their rates beats what I am getting from Shopify's shipping. The price is the same for all the shipping zones. I did one label with them so far and all was good...

I know to create an add to cart link you would use https://domain.com/cart/add?id=VARIANT-ID-HERE that has been shared on this forum. However, my products do not have variants. Is there a way to just add the product ID instead? When I use the product...