Here is the postage calculator on Etsy - you can find it by going to your Shipping Profile page - on the right side there are some links and one is to look up shipping costs.
https://www.etsy.com/shipping/us/price-your-postage?ref=shipping_mgrWith this calculator you put in your zip code and a destination zip code. The dimensions of the package (that would be your envelope not the item) and the weight of the package.
The calculator will give you both Etsy prices and Retail prices side by side.
To see what your calculated shipping looks like to a buyer by putting multiples in your cart there are two ways to do this.
You can sign out of your Etsy ID and go back as a shopper or your can open to your shop in another browser and not sign in.
If you know what it should cost, then you can check to see if that is what is calculating for your buyers.
Remember when you do this that if you have added a shipping handling fee somewhere, you need to add that onto the postage.
Good idea to base your weight on two rather than three. The idea is to get as close as possible. It may not always be right but it should be very close.
I can tell you that I recently had a big sale for 21 items. It was a mix of 5 different types of items from my shop as well. The calculator put them in two boxes and the total weight of all of this was only 8 ounces off from the actual weight I got after boxes up the order. And that 8 ounces could have been the free gifts I added to the package.
My point is, if you get your measurements correct and you have your box or envelopes listed in preferences, CS can work amazing well. It's very important to get your figures right though.
I've made adjustments over time with using CS. It all didn't happen correctly all at once. But it is worth the work.