Install and activate the plugin “WordPress SEO”. It will add options to the bottom of each page (when you’re editing the pages) which will allow you to control the meta description and keywords (among other things) for each page and post of your site.
This allows you complete control as to how your site looks to a search engine… or whether a search engine even sees that particular page.
If you don’t want your “Thank you for making a purchase” page to be found by search engines (or any other page, for that matter) you can use WordPress SEO to turn off search engines accessIbility to that page by setting it to:
Do you have another idea I haven’t listed? Feel free to email me.