Hi there, pffmanager.

Thank you for visiting the QuickBooks Community. I'll be sharing steps to ensure that your customer can pay the invoice with a tip.

The information shared above about clicking the Gear icon and selecting the Sales tab to turn on the Tip (Gratuity) option is only available in QuickBooks Online version. Since you've mentioned that you're using the 2020 Desktop, this is the reason why there is no Gear icon on your home page.

On the other hand, the option to directly add a tip when your customer pays the invoice is currently unavailable in QuickBooks Desktop. You may want to create an item as a tip and add it to the invoice. To start with, you'll have to go to the Lists menu and choose Item List.

Here's how: 

1. Go to the Lists menu then select Item List.
2. Click the Item button and choose New.
3. Select the type of item you want to create under the Type drop-down arrow.
4. For the name, enter something like "Tip"
5. Fill out the item necessary fields.
6. Once done, click OK.

For additional details, you can open this article: Add, edit, and delete items. This also provides information on how you can edit or update the details of your item.

Once done, you can now go to the Invoice page under the Customers menu at the top. Then, add the tip as another line item before sending the entry to your customer. Please note that other users have been tried these steps and it works.

Lastly, you can check out our Accounts Receivable workflows in QuickBooks Desktop. This can help you organize your cash flow, and track sales, receivables, and profitability more accurately.

Please know that you can always get back to this post if you have additional questions with QuickBooks. I'll be around to provide further assistance. Have a good one.