Hello Shannon,
I'm Brian from the Shopify Guru team. :) I hope the day is treating you well.
I've been looking into this for you and I believe that the In-Store Pickup / Click & Collect app is a good start here: https://apps.shopify.com/click-and-collect
The app will allow you to set up a collection option for your store, and allow you to set up a few different delivery locations, which the customer can select which one is closest to them. You could therefore set up the in-store collection shipping rate to apply to all orders (as is recommended by the app developers) and ensure that there are no other shipping options available to orders above a certain weight. This would mean that customers that are ordering the smaller, lighter products would have the option of in-store collection or delivery, whereas those customers ordering the larger products can only select in-store collection. However, it may also be wise to let customers know that the larger products can only be collected, perhaps by displaying this message on the product information page.
The app also comes with a 14 day free trial, so I would suggest giving it a try and see if it suits your needs. :)
If you have any other questions or issues, pleae do not hesitate to get in touch with our gurus at support@shopify.com
Have a lovely day!