How can I calculate shipping discounts without a fully set-up store?

I’m in the preliminary stages of setting up my store and I’m at the point where I’m deciding how products are going to be packed and shipped.

I can pretty easily calculate rates on USPS.com, but that doesn’t tell me what kind of discount I’m going to get through shopify shipping.

I haven’t yet decided if I wanna do flat rate boxes or my own custom package, and I feel like I need to know the discount rates to compare the options.

I have a good idea of what my products will weigh, though I have not added them to my shopify store yet.

Is there a way to accurately calculate shipping costs with shopify shipping discounts without having your store already fully set-up?