Hi everyone! I had difficulty finding a solution for adding a promo code for my online store so that customers can add the promo code during checkout. I finally found out how to do it! It's a total 3 ways to add discounts so I'll show you how to do all 3. So you can see which is best for you.
#1 IF YOU'RE OKAY WITH MANUALLY ADDING THE DISCOUNT:
Dashboard - Items & Services - Items - Discounts
Create a Discount - Insert Designated Discount Name, Amount Type, and Amount
Select Automatic Discount - Discount Rules - Add individual items or item categories (be sure to select Square Online Sites to show on your website)
The Automatic Discount works best if you want the discounted rate to show ONLY in the shopping cart & checkout but NOT on the physical website. However, it will show the name of the Designated discount name on the website on the website before added to the shopping cart.
Once added to the account you can hover over the cart and it will look like this with the discounted rate.
When you're finally ready to check out it will look like this & have a *lighting bolt* directly next to the promo word "LUV" was for this one. 
#2 IF YOU'RE DOING A SALE AND WANT IT TO SHOW A "SALE PRICE" THIS WILL BE A BETTER PICK FOR YOU! IT'S QUICK & EASY!
Dashboard - Items & Services - Items - Item Library
You will see all your items/products here.
Click on the the dots to the right
Click on Update Online Sale Price and then put the Amount or Percentage for the discount
You can also do this by simply Clicking on your Item (as if you was editing it)
Scroll down to Variations & Click Sale Price to update it. A box will pop up next to the right to add the sale price.
I added a 20% discount for this one & it will look like this online.
#3 THIS IS THE FASTEST AND PROBABLY THE MOST USED & INQUIRED IS ADDING A PROMO CODE SO THAT CUSTOMERS CAN ADD IT DURING CHECKOUT.
Dashboard - Customers - Marketing - Coupons
Click on Create Coupon (this is the most flexible and give you tons of options to create a coupon code, fixed discount, percentage off and even free delivery/shipping. You can apply the promo to the entire website or specific items. You can also select conditions so customers can't reuse coupons unless you allow this to.)
Fill out the form to your liking and click Activate and you're all set! You can also share this as a link!
Here's how it would like on the site.
I didn't find out how to add it to for Services but this works wonders for products/items for sure! Hope this helps 🙂
