If you'd like to restrict access to your page or your sales, you'll just have to follow the steps below:
Go to your fundraisers settings by editing your page and opening up the sidebar. Afterwards, press Privacy.
You'll then turn on This page is private. If you're looking to only make the sales private, choose Checkouts Only, otherwise select Entire Page.
Once you've got it turned on, selected what you'd like private, and set the password, make sure to press Publish Changes for this to reflect on your live page.