Action Details
Requirements and how-to articles for itch.io's action:

A sale on itch.io lets you offer any combination of your paid items for a discounted price for any period of time. If you have multiple items in your sale, then they can be bought together as a bundle.

Requirements
You need this to get it done:

Projects
You can select all the projects affected by the sale. To help with picking a rate, a preview price is shown next to each item. Select at least 1 project, but as many as you like. Selecting more than one item will allow for everything to be bought together as a bundle.
Description
This will make up the body of your sale’s page. Tell everyone why you're hosting a sale.
Duration Dates
This is the when your sale will become available. Set the start to the future to have a sale that starts later. Set it to now, or a previous time, to make it immediately available. The end date is when the sale will no longer be available. Use your local time zone when entering dates, it’s accurate down to a second.
Rate
The percentage off the price of each item in the sale. The discount affects all prices, including the base price, individually priced files, and reward prices. You can set the rate to 100% if you want your content to be temporarily free, or if you just want to host a bundle you can set the rate to 0% and provide a bundle price.
Bundle Price
If you have more than one project selected, you can optionally specify a custom buy all price. This will give someone an incentive to buy everything at once if they can save some more.
Sale Goal
If your sale or bundle is part of of a campaign to raise money for an event or cause then you can provide an earnings goal. When enabled, the earnings goal will show on the sale page and on top of your project pages.
Coupon Codes
When you provide a coupon code for a sale a new URL on your account will be created that someone will need to visit to redeem access to the sale. Additionally, you can set a limit on the maximum number of times the coupon is redeemed. Coupon URLs are placed on your subdomain, so they're short and easy to print on cards for events. For example, if you chose the coupon code secret-sale then the address would look like http://username.itch.io/secret-sale. If you can’t think of a name, then click the dice button to randomly generate one. When randomly generating we exclude characters that are commonly mistaken, like 1 and I.
Ownership Requirement
If you're selling multiple products then you can create a sale that is only available to those who own one of your existing items. This is called an ownership requirement. From the sale edit page, select Add condition and choose the game that must be owned for the sale to be active.
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How-To Articles
A list of useful links for you:

0
itch.io Documentation - Hosting a sale or bundle
Information about hosting sales and bundles for paid content on itch.io.
Official
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