Here's a guide I've found useful in my experimentation with Shopify stores in the past: https://help.shopify.com/en/manual/shipping/rates-and-methods/manual-rates
Shipping is definitely one of the trickier pieces of configuration. If the guide itself isn't self-explanatory, can you try entering a weighted rate threshold that starts when the flat rate shipping would become cheaper? For example, let's say the flat rate is $8.00 and you start using it when packages are 13 ounces.
If that doesn't work, let me know. I'd like to see if we can troubleshoot this together.
Yes, but it’s not an exact science! So, I ended up using PirateShip as my shipping app because it gives an additional discount to your shipping costs with Shopify and was easier to manage (IMO) than what I had set up in Shopify. It really does save more money.
PirateShip is free, btw, and I set up my shipment packaging profiles in there! I use bubble mailer envelopes that I purchased on my own, free USPS Priority Mail shipping boxes, and USPS Flat Rate boxes. I figured that at a certain point of weight, it was cheaper for me to send something USPS Flat Rate than USPS Priority Mail (weighted). So, say if something weighed 7 lbs., I would send it Flat Rate because the Flat Rate fee is cheaper than paying Priority Mail at that weight. For me, my “threshold” for USPS Priority Mail (weighted) and USPS Flat Rate is around 6 lbs. I determine what package to ship in and what I am charged when I process an order thru PirateShip.
There are TONS of videos on Youtube for PirateShip if you are curious…and no, I’m not sponsoring them or anything, just a happy customer.