Messages

Hello.
Categories are meant to list products, so AbanteCart shows the "There are no products in this category." message when none are assigned. If you want an info-only page, create it under Design > Content and add it to your menu. Otherwise, you can change that message in Language Definitions (search for the phrase and edit it).
https://abantecart.atlassian.net/wiki/spaces/AD/pages/15466660/Language+Definitions

Geometry Dash

To delete the scam customer: Admin > Sales > Customers, select the customer, then click Remove.

Hi Basara,
Do I need to create a special menu item or layout block for it? And will that content page still support SEO URLs the same way categories do?

You can try placing a test order in a sandbox environment for both PayPal Express Checkout and Square.