Hello Virginia and welcome! You don't have to do anything to offer a discount for multiple items purchased, that is built into calculated shipping.
When you put the dimensions of your item (item with packing material around it but NOT the shipping box) into your item listings, the calculator figures out how to fit your items into boxes so combined shipping is automatic.
For the weight of your item - you weight it in the shipping box - all ready to ship. You need this so if someone only buys one item, they will get charged enough for you to ship it. Yes, this weight gets added to each item purchased, but usually, with a little tweaking sometimes, this works out great.
Most important is to list your boxes in Package Preferences. If you use flat rate or regional rate boxes - you list them here. Also list any other size boxes you use like 6x4x4, 7x7x6, 12x12x8. Most of us only use our own package preferences and uncheck the box to use Etsy common sizes.
This way, when I customer buys something, the calculator will figure out for you how to give the best shipping price. Sometimes it is a flat rate box, and other times it is a regular box - depending on zip code.
But it always figures the cheapest price for the customer.
I sell multiple items all the time. Since getting all of my figures just right, I have not overcharged or undercharged a customer in the last few month. It figures out how to give my buyers the best shipping price whether they buy one item or 6 items.
Start with setting up a shipping profile and be sure if you use First Class, to also include Priority or Parcel Select so if someone buys enough items and it goes past the First Class weight limit - there is another shipping class to jump to. (Don't include Media Mail in a profile unless you sell printed material or books)
Try it will a few items to see how it works. Test by signing out of your Etsy account and going back as a shopper - put multiple items in your cart to see this.
Or go look at my shop now if you want, and put things into your cart (you just delete them later) - but try something like a votive candle set - see how it looks. Add a few and see how the price will level off at certain stages as the calculator uses the best box.