[Richardl]

Currently I set the quantity to 0 for those products without a price to disable add to cart.

However, if the first variant on the list is set to be out of stock, then the rest of variants will appear to be out of stock too even they are stocked.

Vice versa, if the first variant is set to be stocked, then the rest of products will still be able to add to cart even they are out of stock.