Lisa, I wanted to add something this morning.
it took me a long time to get the exact right dimensions for my votive candle sets to fit into the flat rate boxes.
I knew I could fit perfectly fit 8 sets in the medium flat rate box. I tried and tried and finally Mike from the Etsy shipping team helped me and gave me the exact measurements I needed for this to work. It does work now.
But, of course, that is not always the cheapest way to ship. I also have regional A, regional B, large flat rate and priority 12x12x8 as box sizes so the calculated can pick out the cheapest method.
Now that being said, about the 8 fitting perfectly in the medium FR, the calculator still won't put 12 sets in a large FR. They fit in actuality but the calculator just won't do it.
I've been in contact with the Etsy shipping team about it. They are aware it is a problem and say they are trying to fix it. But my point is, sometimes it just can't work for larger numbers of items. You have to do the best you can and get as close as you can.
So far I've only had to refund once because the calculator didn't fit enough into a box. Since I've been using CS since last March, I'd say that's pretty good. I use to refund on most multiple orders.
Keep trying.