Morgan here from the Social Media team at Shopify.
Thanks for reaching out! Depending on whether or not you use carrier calculated rates, or are simply looking to charge a specific *different* flat rate for those products and (buy the shipping label later) there's two ways to accomplish this.
Both do require carrier calculated shipping to be enabled on your account. If you don't have that already feel free to reply here, so we can have that enabled for you.
2. Setting up a weight based rate that will only trigger these products, which in turn will give a different shipping rate than your other products. You can call this rate "Ground/Standard Shipping". However, you will then have to purchase the label while you are fulfilling the order later.
To set up the special weight based rate (that only comes up for certain items) you want to go into your shipping settings and "add a weight based rate". From here, you need to make it applicable only to items over a certain weight. To do this you will have to make sure you choose a weight your cart could never total up to with your other products. IE. make it 500lbs.
Once you have the rate set, you want to change the weight of all the specific products to 501lbs. Now they will all trigger your "Ground/Standard Shipping" rate, while no other product ever will.
One thing to note, this option will not work if you buy your shipping labels within Shopify or use carrier calculated rates at checkout, as you need your weights to be as accurate as possible to the actual item in this case.
If you do use carrier calculated rates, I recommend option 1!