I would include them in the 2020 inventory. Here's why.
THe items are considered "purchased" the instant you lose control of the money used to purchase them. As soon ask you finished entering your credit card information and clicked that "purchase now" button, you instantly lost control of and access to, the money used to pay for those items.
The IRS terms this as "constructive receipt". So you would include the items (as well as the money to pay for those items) on your 2020 tax return.
Now me personally, if you wanted to include it in the 2021 inventory, wouldn't see that as a problem. Technically, you don't receive the items until 2021 and you don't "actually" have them appear on the CC bill until you get the bill in 2021. However, the transaction date on that bill will probably be 2020. So that's where the IRS rules on "constructive receipt" come into play.
