In addition to creating certificates from the administrative part of the site, you can offer your customers to buy them themselves for any amount.
You can embed the certificate order form in any part of the site where it is allowed to use Smarty.
For example, open Shop - Storefront - Pages. Create a page, name it and embed to any part of the content the following code:
{if $wa->shop && method_exists('shopGiftcertificatesPluginHelper', 'form')}{shopGiftcertificatesPluginHelper::form()}{/if}
Here's what you should get:
A form for ordering a gift certificate will appear on the created page.
Appearance of the form and the page will depend on theme design and your CSS styles.