To set up your shop, you'll need to connect your cart software. I recommend PayPal Smart Buttons, Stripe Payment Links or Shopify Lite for your products. You can also create an ecommerce shop with Sellfy and embed it inside of your google site using the full page embed option. Or link to your Etsy Shop or any other marketplace where you sell your products.
If you're using Shopify Lite you may instead use an embed code to embed your Shopify Buy Button directly onto this page. You can embed any payment button from third parties such as Square.
Adding the links are the same process as adding any link to your site or changing links in your buttons. To change a button link double click on the button > Change the name if applicable > add the link to the payment system under "link". Done!
If you would like to add a newsletter link you may do so with any newsletter provider including Mailchimp, Flodesk, Mailerlite, and more - view the tutorial here.