How can I add text to the shop page below the products?

Please ask the provider/vendor of your e-shop theme or plugin. That said, it may just be a CSS issue:

For help with CSS, a live site link is required…if we cannot see the site, we cannot see the stylesheet(s), so we cannot suggest changes.

To discover what CSS is output by your theme, use a web inspection tool such as Firebug: https://getfirebug.com/ , Chrome Inspect Element: https://developers.google.com/chrome-developer-tools/ or Internet Explorer 9+ F12 Dev. Tools: https://msdn.microsoft.com/en-us/library/dd565627(v=vs.85).aspx#csstool

There are others.

When editing CSS, use a Child Theme
https://codex.www.ads-software.com/Child_Themes
Custom CSS Plugin, or Theme provided custom CSS option.
Edits to parent themes are lost on theme update.

Learn CSS: https://www.w3schools.com/css/